Data link controller with flexible multiplexer

ABSTRACT

A data link controller is provided which is suitable for use in ISDN applications, employing bit-oriented protocols. The data link controller can be operated in a time-division multiplexed mode or a non-time-division multiplexed mode. In the multiplexed mode, data in one of up to thirty-one selectable time slots is received by the data link controller under control of a microprocessor. One of the time slots can be selected via the microprocessor which allows a lengthened contiguous number of bits to be received, effectively increasing the reception rate. In the non-multiplexed mode, data is received in a continuous stream. In the multiplexed mode, up to thirty-one time slots are available for transmission of data. One of the time slots can be selected which allows a lengthened continuous number of bits to be transmitted, effectively increasing the transmission rate. In the lengthened configuration, by doubling the length of the time slot, two 64 kbps B-channels can act as a single 128 kbps channel.

CROSS-REFERENCE TO RELATED, CO-PENDING APPLICATIONS

This patent application is a continuation of patent application Ser. No.07/035,683, filed Apr. 3, 1987, now abandoned.

Co-pending application of particular interest to the instant applicationis U.S. Ser. No. 759,622 filed July 26, 1985, on behalf of Alan T.Clark, Hadi Ibrahim and Arthur F. Lange, and entitled "DigitalSubscriber Controller"; U.S. Ser. No. 034,822, filed Apr. 3, 1987, onbehalf of D. Gulick, T. Lawell and C. Crowe, entitled "Data ProtocolController"; U.S. Ser. No. 035,817, filed Apr. 3, 1987, on behalf of D.Gulick, T. Lawell and C. Crowe, entitled "Packet-At-A-Time Reporting ina Data Link Controller"; U.S. Ser. No. 035,684, filed Apr. 3, 1987, onbehalf of D. Gulick, T. Lawell and C. Crowe, entitled "EnhancedUniversal Asynchronous Receiver-Transmitter"; and U.S. Ser. No. 035,687,filed Apr. 3, 1987, on behalf of D. Gulick, entitled "Dual-Port TimingController", all assigned to the assignee of the instant application.

FIELD OF THE INVENTION

This invention relates to data communication networks that comply withinternational protocols and, more particularly, of the second level ofprotocol, a data link controller having user-accessible registersmonitoring status and controlling operation of the controller,especially, a register which permits selection of non-multiplexed ormultiplex operation.

BACKGROUND OF THE INVENTION

Data link controllers (DLCs) are used in integrated services digitalnetworks (ISDNs) to provide a high-speed 192 kilo-bit per second (KBPS)serial interface to a telecommunications network. Various time-divisionmultiplexed channels are employed for data transmission in ISDN, such asa signaling (D-channel) and two voice/data (B-channels). In order toprovide reliable error-controlled data transmission, members ofBit-Oriented Protocol (BOP) families are used.

The ISDN protocols specify a data format that multiplexes three, 64 KBPSchannels on the 192 KBPS. Furthermore, the ISDN DLC's support twoindependent "B" channels each having a 64 KBPS data transfer rate.

SUMMARY OF THE INVENTION

A data link controller (DLC) is provided which is suitable for use inISDN applications, employing bit-oriented protocols. In one aspect ofthe DLC of the instant invention, the DLC can be operated in atime-division multiplexed mode or a non-multiplexed mode. In themultiplexed mode, data in one of up to thirty-one selectable time slotsis received by the DLC under control of a microprocessor. One of thetime slots can be selected via the microprocessor which allows alengthened contiguous number of bits to be received, effectivelyincreasing the reception rate. In the non-multiplexed mode, data isreceived as a continuous stream. In the multiplexed mode up tothirty-one time slots are available for transmission of data. Time slotselection is user-controlled via the microprocessor. One of the timeslots can be selected which allows a lengthened continuous number ofbits to be transmitted, effectively increasing the transmission rate. Inthe lengthened configuration, by doubling the length of the time slot,two 64 KBPS B-channels can act as a single 128 KBPS channel.

Control of the mode of operation of the DLC of the present invention isprovided by various Status and Control Registers accessible by the uservia a microprocessor. Particular registers have bit positions monitoringstatus conditions in such a manner that the most-probable one of a setof conditions occupies the least-significant bit position, while theleast-probable condition occupies the most-significant bit position. Inthis aspect of the invention, a user can efficiently test conditions;both in terms of real-time monitoring and programming ease.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an illustrative case of an ISDN Protocol Controller (IDPC)of the instant invention in a terminal adapter (TA).

FIG. 2 is a block diagram of the IDPC 10 of the instant invention.

FIG. 3 is a functional block diagram of the IDPC 10 focusing on theinterrelationship between the DLC 52 and the remaining portions of theIDPC.

FIG. 4 is a block diagram of the transmitter portion of the DLC 52 ofthe IDPC 10.

FIG. 5 illustrates the structure of the transmitter first-in, first-out(FIFO) 100 of the DLC 52.

FIG. 6 illustrates the serial-to-parallel shift register 110 of thetransmitter 102 portion of DLC 52.

FIG. 7A illustrates the structure of the Zero Bit Insertion Unit 124 ofthe DLC 52 transmitter 102.

FIG. 7B illustrates the structure of the Flag/Abort Insertion Unit 134of the transmitter 102 portion of DLC 52.

FIG. 8 is a block diagram of the transmitter portion of the Serial BusPort 104.

FIG. 9 shows the timing of the transmitter portion of SBP 104.

FIG. 10 is a block diagram of the receiver portion of the DLC 52 of theIDPC 10.

FIG. 11 is a block diagram of the receiver portion of the Serial BusPort 104 of DLC 52.

FIG. 12 shows the timing of the receiver portion of SBP 104.

FIG. 13 is a block diagram of the Flag/Abort Detection Unit 214 of thereceiver 108 portion of DLC 52.

FIG. 14 is a block diagram of the Zero Bit Deletion Unit of element 218of the DLC receiver 108.

FIG. 15 is a block diagram of the Short Frame Byte Counter 260 ofelement 218 of the DLC receiver 108.

FIG. 16 is a block diagram of the Serial-to-Parallel Shift register 208and 210 and related element of the DLC receiver 108.

FIG. 17 is a block diagram of the address detection unit 226 of the DLCreceiver 108.

FIG. 18 illustrates the structure of the receive FIFO 106 within the DLC52.

FIG. 19 is a state diagram of the operation of the DLC 52 transmitter102 portion.

FIG. 20 is a state diagram of the operation of the DLC 52 receiver 108portion.

FIG. 21 is a functional block diagram of the UART 54 employed in theIDPC 10 of the present invention.

FIG. 22 is a block diagram of the parity checker and special characterrecognizer 412 of the UART 54 employed in the IDPC 10.

FIG. 23 is a block diagram showing interconnection of the Dual-PortTiming Controller (DPTC) 56 of the IDPC 10 of the instant invention to ahost processor and a local processor.

FIG. 24 is a functional block diagram of the DPTC 56 of the IDPC 10 ofthe instant invention.

FIGS. 25A and 25B are timing diagrams showing the timing relationshipamong the control signals received by the DPTC 56 and then subsequentlygenerated thereby.

FIG. 26 illustrates the interprocessor interrupt scheme employed by theIDPC 10 of the present invention.

FIG. 27 is a diagram of the four-stage "delayed status" apparatusemployed for the Receive Frame Status and Receive Byte Count Registerand the Receive Link Address Bit Field of the Interrupt Source Registerof the DLC 52.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

With reference to FIG. 1, an illustrative use of an integrated DataProtocol Controller (IDPC) 10 in a terminal adapter (TA) is shown. ADigital Subscriber Controller (DSC) 12 such as that described in therelated, cross-referenced, co-pending patent application U.S. Ser. No.759,622 filed July 26, 1985 and entitled "Digital Subscriber Controller"assigned to the assignee of the instant application, which applicationis incorporated herein by reference, is shown in FIG. 1 connected by abidirectional bus 14 to the IDPC 10: the serial port of DSC 12 to aserial bus port of the IDPC 10. (The numeral "4" appears adjacent to bus14 indicating that four signals are conducted in parallel on bus 14;this notational practice is used henceforth in the various drawingsemployed in describing the invention.) As described in the related,co-pending application, the DSC 12 interconnects the TA at an "S"interface to network terminating (NT) equipment. As such, the various D-and B-channels are demultiplexed off the telecommunications network andpassed to the TA, and conversely, the separate B- and D-channels aremultiplexed for transmission over the network. A bidirectional data bus16 interconnects the DSC 12 and the IDPC 10; the data bus 16 carries B-and D-channel information between these elements as well as between amicroprocessor 18, a read-only memory (ROM) 20 and random access memory(RAM) 22.

The microprocessor 18 generates lower-byte address signals conductedalso on bus 16 (time-division multiplexed with the data signals) to anaddress latch 24 and generates upper-byte address signals conducted on abus 26 to the latch 24. The latch then conveys this address signal via abus 28 to the IDPC 10, the DSC 12, the ROM 20 and the RAM 22. Clock(CLK), chip select (CS), read (RD) write (WR) signals generated by themicroprocessor 18 are conducted to the IDPC 10, the DSC 12, the ROM 20and RAM 22 via signal lines 30, 32a-d, 34 and 36, respectively. Anaddress latch enable (ALE) signal generated by microprocessor 18 isconducted to an enable (E) terminal of the address latch 24 via a signalline 36. The meaning and operation of these signals is well known tothose skilled in the art and will not be described herein.

Finally, a data link controller interrupt (DLCINT) signal and aUniversal asynchronous receiver transmitter (UART) interrupt (UARTINT)signal are conducted via signal lines 38 and 40, respectively, from IDPC10 to INT0 and INT1 terminals of microprocessor 18. Their meaning andoperation will be described hereinafter.

Two line drivers 42 and 44 are shown connected to the IDPC 10 providingfull duplex serial data transmit and receive functions, respectively,coupled to a UART section of IDPC 10. Various control signals arerequired to be conducted between the elements illustrated in FIG. 1, aswill be understood by those skilled in the art, but are not shown in theinterests of clarity.

The IDPC 10 is controlled by internal status and control registers whichare read and written by software running on the microprocessor 18("local processor") connected to the IDPC 10. While the IDPC registerscan be mapped into either memory 20 or 22 or to input/output devices.The IDPC 10 registers occupy a 64 byte block located in themicroprocessor 18 address space. The starting address of this block isdetermined by address decode logic external to the IDPC 10 which is usedto generate the IDPC chip select signal.

In systems containing more than one microprocessor (e.g., PersonalComputer with a FPU ("host processor") and a local processor), only thelocal processor can access the IDPC registers. Although the IDPC 10contains bus arbitration hardware to allow two processors to share RAM22 on the IDPC external bus 16, there is no corresponding built-inarbitration to allow a processor other than the local processor toaccess the IDPC registers. Similarly, the IDPC 10 does not provide amechanism for the local processor to access devices (including host RAM)not connected to the IDPC external bus 16.

A "host" processor can control IDPC operation indirectly, however, byissuing requests to the local processor 18. This is accomplished via BusArbitrtion (memory sharing) and Interprocessor Interrupt facilitiessupplied by the IDPC 10. A description of this arrangement is givenbelow in connection with FIG. 23.

Briefly, the host processor writes a command (e.g., "Send Data on theB-Channel" and associated parameters to a set of contiguous locations ofRAM 22 on the IDPC external bus 16. This portion of RAM 22 (referred toas a "mailbox") is no different from the rest of the RAM on the IDPCexternal bus 16 except that the local processor 18 software "knows" thatcommands from the host processor will be placed at that particularaddress. The operation of the IDPC Bus Arbitration that allows eitherprocessor to access the mailbox is totally transparent to software. Thehost processor uses the IDPC Interprocessor Interrupt facility to notifythe local processor that there is a command present in the mailbox. Thelocal processor notifies the host processor of command results andstatus in a similar fashion, writing to pre-arranged locations in RAM onthe IDPC external bus and interrupting the host processor using the IDPCInterprocessor Interrupt facility.

The four major elements internal to the IDPC 10 are shown in FIG. 2comprising microprocessor interface (MPI) 50, a data link controller(DLC) 52, a universal asynchronous receiver-transmitter (UART) 54 and adual-port timing controller (DPTC) 56. A signal line 57 conducts theclock signal generated by the MPI 50 to the DLC 52, the UART 54 and theDPTC 56. Bidirectional Busses 58, 60 and 62 interconnect all majorblocks of the IDPC 10; the DLC 52, the UART 54 and the DPTC 56. Bus 58conducts address signals received on a 6-conductor address signal lineby MPI 50. Bus 60 conducts data signals received at an 8-conductor datasignal line by MPI 50. Bus 62 conducts control signals generated by MPI50 upon reception thereby of chip select (CS), write (WR), read (RD)signals and power down/reset (PD, RESET) signals. A more completedescription of the UART 54 and the DPTC 56, including the various signallines shown in FIG. 2 will be given in connection with FIGS. 21-25.

The MPI 50 connects the DLC 52 and UART 54 to the externalmicroprocessor 18. The design and construction of the MPI 50 isconventional and understood by those skilled in the art. Accordingly, itwill not be elaborated on herein. As will be explained hereinbelow, theDLC 52, UART 54, and DPTC 56 have user accessible registers. A 64 byteaddress space is subdivided by the MPI 50 into a 31 byte UART space, a32 byte DLC space, and a 1 byte DPTC space. The address decoding of theindividual spaces is performed internal to the UART 54, DLC 52, and DPTC56. The 64 byte space is allocated as follows:

    ______________________________________                                        ADDRESS              USE                                                      ______________________________________                                        00-31                DLC 52                                                   32-62                UART 54                                                  63                   DPTC 56                                                  ______________________________________                                    

The DLC portion 52 of the IDPC 10 has the task of providing a fullduplex interface (simultaneous transmit and receive) between the SerialBus Port (SBP) 104 and three internal parallel busses 58, 60 and 62 ofthe IDPC 10. Through the use of separate 16-byte receive and transmitfirst-in first-out FIFOs buffers and two external direct memory access(DMA) channels, the DLC 52 provides movement of data to and fromexternal memory 20 and 22 and the SBP. The DLC performs low level (ISOlayer 2-) bit oriented protocol processing on this data. The majorprotocols supported are SDLC, HDLC, LAPB (X.25), and LAPD.

FIG. 3 represents a functional block diagram of the IDPC 10 highlightingthe major functional blocks of the DLC 52. The MPI 50 is showninterconnecting the external data and address busses 16 and 28 (FIG. 1)as well as the control lines 30, 32a,, 34, 36 and 38 with the internalbusses 58, 60 and 62 (FIG. 2). The DPTC 56, UART 54 and DLC 52 sectionsof the IDPC 10 are connected to busses 58, 60 and 62. The DLC 52 portionof the IDPC 10 is shown in FIG. 3 as comprising five major functionalblocks. A transmit first-in, first-out (XMIT FIFO) register 100connected to the internal busses 58, 60 and 62. A transmitter 102 isconnected to XMIT FIFO 100 and to the internal busses 58, 60 and 62 andto the Serial Bus Port (SBP) 104. A receive first-in, first-out (RECVFIFO) register 106 is connected to the internal busses 58, 60 and 62. Areceiver 108 is connected to RECV FIFO register 106 and to the internalbusses 58, 60 and 62 and to the SBP 104. The transmitter 102 andreceiver 108 each include status, command and control registers. Allprogrammable registers and a portion of the FIFOs called data registerscan be accessed via the busses 58, 60 and 62. These registers are mappeddirectly into microprocessor 18 memory space, and are described indetail hereinafter.

The Serial Bus Port (SBP) 104 receives a serial clock (SCLK) signal, aserial frame synchronization (SFS) time-multiplexed with a transmitclock (XMITCLK) and generates a serial bus output (SBOUT) signal. Thedescription of operation of DLC 52 transmitter and receiver portionsrequires an understanding of bit-oriented protocols (BOPs) such as SDLC,HDLC, LAPB (X.25) and LAPD.

Bit oriented protocols provide a set of rules and techniques thatfacilitate the transfer of data over a communications network. Thisdiscussion will not concern the workings of the upper level of theprotocols--sequence numbers, acknowledges, and the like--since these arethe responsibility of the software that runs on the local processor 18.The present description concentrates on the aspects of the protocolsthat affect the hardware of the DLC 52.

The BOPs call for transmission of data in packets. Packets are delimitedby unique Flag characters and contain an address, some controlinformation, the data itself, and an error detection code. The addressidentifies the sender or the receiver of the data. The controlinformation is used by higher levels of the protocol to manage the flowof data. The data, which is contained in the information field, is userinformation. Packets that are used for protocol control often omit theinformation field (this is the only optional field). The error detectioncode is a Cyclical Redundancy Check (CRC) and DLC the 52 uses theCCITT-CRC code. In addition to addresses, control, data, and errorchecking, the BOPs employ such mechanisms as Flags, Bit Stuffing, andAbort characters. The following section is a glossary of BOP terms andfunctions. These items will be used throughout the description of theDLC 52.

In the bit oriented protocol environment data is transmitted in frames.Protocols such as SDLC, HDLC, LAPB (X.25), and LAPD share the same basicframe format:

    __________________________________________________________________________                              FRAME                                               FLAG ADDRESS                                                                             CONTROL                                                                              INFO    CHECK  FLAG                                              (1 or 2                                                                             (1 or 2                                                                              (OPTIONAL)                                                                            SEQUENCE                                            01111110                                                                           bytes)                                                                              bytes)         (16 bits)                                                                            01111110                                     __________________________________________________________________________

The eight bit Flag character is identical for all of the above mentionedprotocols. It is exactly 01111110. Its bit pattern is unique within apacket because the "bit stuffing" technique (described later) used doesnot allow six contiguous ONEs to be present in the packet portion of aFrame. The Flag character can perform three functions: as an OpeningFlag, as a Closing Flag, and as an inter-packet fill character.

The Opening Flag is defined as the last (perhaps only) Flag prior to anon-Flag, non-Abort character. (The Abort character is defined below.)All valid packets must begin with a Flag. The Opening Flag indicates thebeginning of a packet. When Flags are being used as interframe fillcharacters, a non-Flag, non-Abort character must be received before thepreceding Flag can be identified as an Opening Flag.

The principal difference between the lower levels of the various BOPs isthe address field. All addresses are of an integer number of bytes inlength. In general an address can be one, two, or N bytes long.

The length of an N byte long address is determined by the value of theleast significant bit in each byte of the address. This bit, called theExtended Address bit (EA), identifies the last byte of the address. Allof the bytes of an N byte long address will have the EA bit cleared to aZERO except the last byte of the address. The presence of an EA bit setto a ONE indicates that the byte is the last byte of the address. Thelength of the address field affects the detection of a Short Frame.

In some protocols the second bit (bit 1) of the first byte of theaddress is used to indicate whether the frame is a command or aresponse. This bit, called the command/response bit (C/R), can be eithera one or a ZERO without invalidating the address.

The control field immediately follows the address field. The DLC 52treats the control field as packet data. That is, the DLC 52 does nottake any action in response to the contents of the control field. Thecontrol field can be either one or two bytes long. The length of thecontrol field has an impact on the detection of a Short Frame.

The information field, when present, follows the control field andprecedes the Frame Check Sequence. The information field contains thedata that is being transmitted between users. The information fieldcontains an integer number of bytes, and can be up to 64K bytes long(minus address and control lengths) for the IDPC 10.

The Frame Check Sequence (FCS) is a 16 bit word that is produced by aCRC generator and checked by a CRC checker. Mathematically it is theONEs complement of the sum [modulo 2] of the following: The remainder ofX^(K) [X¹⁵ +X¹⁴ +X¹³ +. . . +X² +X+1] divided [modulo 2] by thegenerator polynomial X¹⁶ +X¹² +X⁵ +1, where K is the number of bits inthe frame existing between, but not including, the final bit of theOpening Flag and the first bit of the FCS, excluding bits inserted fortransparency.

    --AND--

The remainder after multiplication by X¹⁶ and then division [modulo 2]by the generator polynomial X¹⁶ +X¹² +X⁵ +1, of the content of theframe, between but not including the last bit of the Opening Flag andthe first bit of the FCS, excluding bits inserted for transparency.

The Closing Flag is the last field in the frame. It indicates the end ofthe frame and signals that the FCS should be checked.

A packet is a frame minus the Opening and Closing Flags.

When frames are not being transmitted over the link provided by DLC 52,the link is said to be idle. When the link is idle the DLC transmitter102 can be programmed by microprocessor 18 to send an all ONEs pattern.This is referred to as a Mark Idle (MI) condition. Specifically, an MIis defined as at least 15 contiguous ONEs.

Prior to and between frames, back to back Flags can be transmitted overthe link. This is referred to as a Flag Idle (FI) condition and isselected by microprocessor 18 program control.

The DLC receiver 108 is said to be In-frame when it is enabled and thefirst non-Flag, non-Abort character is received after the receipt of atleast one flag. In-frame is valid until the Closing Flag is detected, anAbort character is received or an error is detected. The DLC transmitter102 is said to be In-frame from the time that it starts to send anOpening Flag until the last bit of the Closing Flag has beentransmitted, assuming that the transmitter 102 is not commanded to sendan Abort sequence.

The DLC receiver 108 or transmitter 102 is said to be Out of Frame anytime it is enabled and no In-frame.

Any pattern of at least seven contiguous ONE bits is said to be an Abortcharacter. An Abort character is a physical entity, not to be confusedwith the Abort condition--which is an action. The Abort condition,simply called an Abort, is described below. It is important to note thatthere is a subtle difference between an Abort character and a Mark Idlecondition. Back to back Abort characters do not necessarily constitute aMark Idle condition. A repeating pattern of seven ONEs followed by aZERO (111111101111111011111110. . . ) is a series of Abort characters,but not a Mark Idle. The DLC sends at least one "01111111" whencommanded by microprocessor 18 to send an Abort.

The Abort condition is an action that takes place in response to thedetection of an Abort character while the DLC receiver 108 is In-frame.An Abort causes the termination and discarding of the packet beingreceived. Aborts are asynchronous events in that they can be detected onbit boundaries as well as byte boundaries.

Zero Bit Insertion/Deletion, often referred to as Bit Stuffing, is atechnique used to provide data transparency. By this is meant a methodby which packet data patterns are prevented from appearing as Flags,Aborts, or Mark Idles when they appear in the received data stream.Flags, Aborts, and the Mark Idle condition all consist of six or morecontiguous ONE bits. The Bit Stuffing technique examines the contents ofa packet to be transmitted by transmitter 102, on a bit by bit basis,(from the first bit after the Opening Flag to the last bit of the FCS)and inserts a ZERO in the bit stream after any pattern of fivecontiguous ONEs, thus insuring that six or more ONEs do not appear inthe data stream. The receiver 108, in turn, examines the data stream andremoves the inserted ZEROs that follow five contiguous ONE bits. Theimplication of this is that Flag, Abort, and Mark Idle generation anddetection must take place on the network side of the zero insertion anddeletion units.

The BOPs specify minimum lengths for valid packets. This is usuallyfour, five, or six bytes. Any frame that is received with fewer thanthis legal minimum number of bytes in its packet is called a Short Frameand is considered an error which should be discarded.

On a theoretical basis a frame can be any length greater than thespecified minimum. As a practical matter, however, a maximum packetlength must be set to prevent FIFOs 100 and 106 buffer overrun. Thislength is dynamic, and can vary on a data call by data call basis. Anyreceived frame whose packet exceeds this maximum length is referred toas a Long Frame, and is considered an error. The detection of a LongFrame error takes place as soon as the maximum legal number of bytes hasbeen exceeded, not when the entire frame has been received.

If a Closing Flag is detected and a non-integer number of bytes has beenreceived, (that is to say that the character preceding the Flagcontained fewer than eight bits) a Non-Integer Number of Bytes Errorcondition exists.

The bytes are transmitted in ascending numerical order; inside a byte,the least significant bit (bit 0) is transmitted first. However, the FCSis numbered and transmitted in reverse to this convention.

As shown in FIG. 3, the transmitter sections 100 and 102 of the DLC 52reside between the off-chip memory 22 and the serial port 104 to thedata communications network. The microprocessor 18, under softwarecontrol, builds a data block in memory 22 that contains the address,control, and information portions of a packet. This block of data ismoved, byte at a time, into the transmit FIFO 100 via either DMA orprogrammed I/O. The transmitter section 102 sends the Opening Flag,transmits the block of data, generates and sends the FCS (if selected),and transmits the Closing Flag. The polarity of the data stream can beinverted as it is being transmitted, if desired. Between packets thetransmitter section 100 can be programmed to output an all ONEs pattern(Mark Idle), or back to back Flags (Flag Idle). The transmission of apacket can be terminated by sending an Abort sequence in response to theSend abort bit being set in the Command/Control Register (bit 0).

With reference to FIG. 4, a block diagram of the transmitter sections100 and 102 of the DLC 52, shows the 16-byte XMIT FIFO 100interconnecting the internal busses 58, 60 and 62 and aparallel-to-serial shift register 110. Status and control registers 112within transmitter 102 are connected to busses 58, 60 and 62. Controlsignals are conducted to and from the various elements of thetransmitter 102 and the status and control registers 112 on lines notshown in FIG. 4. Appendix A contains a full description of DLC statusand control register.

Data signals generated by shift register 110 are conducted on line 114to a 2:1 multiplexer (MUX) 116 which also receives via a signal line 118signals generated by a cyclic redundancy code (CRC) generator 120 via aninverting amplifier 122. The CRC generator 120 receives the data signalson line 14 generated by shift register 110. The signals selected by 2:1MUX 116 are conducted to a Zero Bit Insertion unit 124 via a signal line126. The Zero Bit Insertion unit 124 generates shift clock signalsconducted to shift register 110 and CRC generator 120 via a signal line128.

A 2:1 multiplexer (MUX) 130 receives the signals generated by Zero BitInsertion unit 124 via signal line 132 and receives a signal generatedby a Flag, Abort Generator 134 via signal line 136. The signal selectedby 2:1 MUX 130 is conducted to the Serial Bus Port 104 via a signal line138. SBP 104 generates timing signals conducted on a signal line 140 tothe Zero Bit Insertion unit 124 and the Flag, Abort generator 134.

The major elements of the transmitter sections 100 and 102, shown inblock form in FIG. 4, will be described in greater detail in conjunctionwith FIGS. 5-9. With reference to FIG. 5, the transmit FIFO 100 consistsof a FIFO buffer 150, a Transmit Byte Count Register 152, a TransmitByte Counter 154, a DMA Data Request Generation logic 156 and thresholdcomparison logic 158.

The Transmit Byte Counter Register 152 is a read/write register and isconnected to the internal busses 58, 60 and 62 and receives a LOADsignal generated by the FIFO buffer 150 via a signal line 162. It alsoreceives a SEND ABORT signal generated by Flag, Abort generator 134 viaa signal line 164. The transmit Byte Counter 154 also receives the LOADand SEND ABORT signals and is connected to register 152 via a bus 166.Counter 154 receives a COUNT signal generated by FIFO buffer 150 viasignal line 168 and a SET TAG signal generated by counter 154 isconducted to FIFO buffer 150 via signal line 170. A Transmit ByteCounter equals zero (TBC=0) signal generated by Counter 154 is conductedto Data Request Generation Logic 156 via line 170 and a thresholdreached (THLD RCHD) signal generated by the threshold comparison logic158 is conducted to logic 156 via line 172. Logic 158 is connected tothe FIFO buffer 150 via a signal line 174, and is also connected to theinternal busses 58, 60 and 62.

The FIFO buffer 150 is 16 bytes deep and nine bits wide (8 data bits andone tag bit, the tag indicating "the last byte of a transmit packet").Data is loaded into the "bottom" of the buffer termed a FIFO DataRegister 160 programmed by the microprocessor 18, via I/O or DMA. Datais moved therefrom up to the highest unfilled FIFO location on buffer150 at a clock rate supplied by the microprocessor 18.

Data is unloaded from the buffer 150 by the Parallel to Serial ShiftRegister 110. Both loading and unloading operations are asynchronous tothe microprocessor clock. The buffer 150 is cleared on Reset or when anAbort is transmitted. Associated with the buffer 150 is a ThresholdReached signal generated by the threshold comparison logic 158. Thesignal is active whenever the number of bytes in the buffer is at orbelow the threshold level stored in a FIFO Threshold Register withinstatus and control registers 112. The Threshold Reached signal isconducted to the Data Request Generation logic 156 as an indication thatthe buffer 150 should be reloaded. The Threshold Reached signal isreported in the FIFO Status Register, bit 2. A Data Request One (DRQ1)interrupt signal is generated by Data Request Generation Logic 156 whenthe level in the FIFO buffer 150 falls to the threshold level. DRQ1 isconducted to an external terminal of IDPC 10.

A user accessible location of the FIFO buffer 150 is termed the DataRegister 160. The buffer 150 generates a status signal that reflectswhether or not the Data Register is empty (available). This signal,Buffer Available, is reported in bit 3 of the Transmit FIFO StatusRegister (App. A). The bit is set anytime the Data Register 160 isempty, and cleared when the Data Register 160 is written AND the bufferlocation directly above the Data Register 160 is full. This laststipulation on the clearing of the Buffer Available bit prevents the bitfrom clearing, only to be reset when the Data Register is emptied on thenext clock cycle.

If the Parallel To Serial Shift Register 110 attempts to unload a bytefrom an empty buffer, an Underrun condition exists. This causes an errorto be reported via bit 4 of the Transmit FIFO Status Register. Amaskable interrupt is generated by the setting of this bit. In responseto the Underrun, an Abort is generated elsewhere in the DLC 52. Thiscauses the Transmit Byte Count Register 152 and the Transmit ByteCounter 154 to be reset to ZERO as well as the FIFO buffer 150 to becleared.

The Transmit Byte Count Register (TBCR) 152, holds the length of thepacket to be transmitted (exclusive of the Opening Flag, FCS, andClosing Flag). This value is loaded into the TBCR 152 by microprocessor18 software via the Internal Bus 58, 60 and 62. The TBCR 152 is clearedwhen the DLC 52 is reset or when an Abort is transmitted. When the DLCtransmitter 102 is Out of Frame the contents of the TBCR 152 is loadedinto the Transmit Byte Counter 154 at the same time it is written intothe TBCR 152. The contents of the TBCR is also loaded into the TransmitByte Counter when the last byte of a packet (tagged as such) is removedfrom the FIFO buffer 150. (This also insures that the correct value isloaded into the TBC 154 if the TBCR 152 is updated while the transmitteris In-frame). Loading of the TBCR 152 is delayed if the TBCR 152 isbeing written at this time.

The Transmit Byte Counter (TBC) 154 is used to count the number of bytesloaded into the buffer 150 for a given packet. The TBC 154 is loadedfrom the Transmit Byte Count Register 152 and decremented once for eachbyte loaded into the buffer 150. When the contents of TBC 154 reachesZERO, the byte that caused the TBC 154 to reach ZERO is tagged as thelast byte of the packet. This tag is created by setting the ninth bitposition of that byte to a ONE. Recall that the buffer 150 159 is ninebits wide. The ninth bit position is used to hold this tag, whichtravels with the last data byte through the buffer 150. The tag is usedto load the TBC 154 from the TBCR 152, and indicated the end of a packetto the DLC 52.

The Data Request Generation Logic 156 generates the Data Request (DRQ1)signal. DRQ1, when active, indicates to the DMA that the buffer 150 isavailable for the loading of data. The DRQ1 signal is activated when theTBC 154 is not ZERO -AND- the FIFO buffer 150 does not contain a taggedbyte -AND- the level in the buffer 150 is below the programmed threshold(bits 3-0 of the FIFO Threshold Register). DRQ1 remains active until theTBC=0 signal is generated -OR- the buffer 150 becomes full. In thismanner when the level in the buffer 150 falls to the threshold and thereis more data in the packet to be loaded into the buffer 150, DRQ1 willgo active. DRQ1 will remain active until the buffer 150 is completelyfull or the last byte of the packet is loaded into the buffer. Thisinsures that there can never be data from more than one packet in thebuffer 150 at any one time since even if the TBCR 152 is written beforethe last byte of the packet has been transmitted, DRQ1 will remaininactive until the tagged byte is removed from the buffer. DRQ1 isindirectly made inactive by Reset since the TBC 154 is cleared to ZEROon Reset. DRQ1 will become active in this case as soon as the TBCR 152is written (non-ZERO).

With reference to FIG. 6, data is conducted from the Transmit FIFO 100to the eight bit shift register 110, one byte at a time. Each byte isshifted out of the shift register 110 serially by reception of the ShiftClock, which is provided by the Zero Bit Insertion Unit 124. The signalsgenerated by shift register 110 is conducted to the CRC generator 120,and to the 2 to 1 multiplexor 116.

The shift register 110 is responsible for generating the load controlsignals that effect the movement of data from the FIFO buffer 150 intothe shift register 110. The first load is enabled automatically after anIDPC or DLC Reset or fter after the last byte of a frame (tagged assuch) has left the shift register 110. The load will take place as soonas a byte of data reaches the top of the FIFO buffer. Thereafter, theshift register 110 will attempt to load itself every eighth shift clockcycle by virtue of the divide-by-eight counter 110a which generates aLOAD signal conducted to register 110, until the last byte of the packethas been loaded.

The counter 110a is held in Reset when the DLC 52 is in Reset, thetransmitter 102 is Idle, transmitting an Abort, a Flag, or the FCS. Whenthe first byte of a packet reaches the top of the FIFO buffer 150 it isautomatically loaded into the shift register 110. This action is theindication to the Zero Bit Insertion Unit 124 to gate the shift clockthrough to the shift register 110, and remove the Reset control from thedivide by eight counter 110a.

Data is shifted out of the shift register 110 on the falling edge of theShift Clock. The Shift Clock is generated by the Zero Bit Insertion Unit124 and runs at approximately the transmitter data rate: the Zero BitInsertion Unit 124 increases the length of the shift clock cycle thatfollows five consecutive ONE bits, by a full bit time. This allows for aZERO to be inserted in the data stream (the insertion takes place pastthe Serial to Parallel Shift Register 110). With the exception ofchanging the duty cycle of an occasional Shift Clock cycle, the zeroinsertion process has no effect on the operation of the shift register110. The Shift clock is synchronous to the transmitter clock (XMITCLK).When the transmitter 102 is not shifting data out of the shift register(i.e., when the transmitter is either in Reset, Idle, sending a Flag,sending an Abort, or sending the FCS), the shift clock is blocked at theZero Bit Insertion Unit.

The shift register 110 is cleared by a Reset or an Abort.

Referring again to FIG. 4, the CRC generator 120 produces a 16 bit wordreferred to as the Frame Check Sequence (FCS). The mathematical equationdescribing this operation is described hereinabove. Design andconstruction of the CRC generator 120 is well known to those skilled inthe art and will not be further described herein.

The signals generated by the Parallel to Serial Shift Register 110 andthe CRC generator 120 are conducted to the Zero Bit Insertion Unit 124via the 2 to 1 multiplexor 116. During the data portion of a packet, theaddress, control and information fields will be referred to as the"data", the multiplexor 116 passes data from the shift register 110.After the last bit of the data portion of the packet has been shiftedout of the shift register 110, the FCS is passed out of the CRCgenerator 120 if the CRC generator is enabled.

A 2:1 MUX 116 control signal causes 2:1 MUX 116 to select the data pathfrom the Parallel to Serial Shift Register 110, except when the FCS isactually being transmitted.

To maintain data transparency the DLC transmitter 102 will examine theframe content between the Opening and Closing Flag (including theaddress, control, information, and FCS fields) and will cause the ZeroBit Insertion Unit 124 to insert a ZERO bit after all sequences of fivecontiguous ONEs. This is done to ensure that the Flag and Abortsequences are not simulated in the data stream. Additionally, the ZeroBit Insertion Unit 124 generates the Shift Clock, which is used by theParallel to Serial Shift Register 110, the CRC Generator 120, and a MUXcontrol signal generator (not shown). With reference to FIG. 7A, theZero Bit Insertion Unit 124 consists of a three bit counter 176 andrelated logic.

Three-bit counter 176 receives at a count enable input the data streamon the signal line 126 generated by 2:1 MUX 116. The transmit clock(XMIT CLOCK) signal is conducted to counter 176 as well as to a clockstretcher circuit 178. The resulting counter signal is generated bycounter 176 on a signal line 180 when conducts the signal to clockstretcher 178, to an input of an AND gate 182 and to a complement inputof an OR gate 184. AND gate 182 also receives the data stream on signalline 126 as does OR gate 184 at a second complement input. OR gate 184generates a signal applied at a load-to-zero input of counter 126. TheAND gate 182 generates a signal conducted on line 132 to 2:1 MUX 130.

The counter 176 is automatically cleared to ZERO when it is reset. Asthe data stream is received on line 126, the counter 176 is incrementedeach time a ONE bit is detected and reset (to ZERO) when a ZERO bit isdetected. In this way the counter 176 will only count up to five whenfive consecutive ONEs are encountered. When the counter 176 reachesfive, three actions are taken: the data input to 2:1 MUX 130 is forcedLOW for one cycle of the transmit clock by AND gate 182, the next ShiftClock cycle is lengthened by one cycle time by clock stretcher 178, andthe counter 176 is reset. (Note that for purposes of description allaction is shown related to the same clock edge, such as: the same clockedge that shifts the fifth ONE bit to the Zero Bit Inserter is shownclocking it into the counter--obviously, this creates race conditions.These can be avoided by using delayed clocks where appropriate as willbe appreciated by those skilled in the art.) By forcing the data inputto a LOW, a ZERO is inserted in the data stream since the transmit clockis still running. By stretching the Shift Clock, the shifting of thenext bit of data (from either the Parallel to serial Shift Register 110or the CRC Generator 120) is held off for one bit time--making room forthe inserted ZERO. The counter 176 is held in Reset whenever thetransmitter is sending Flags, Aborts, or is Mark Idling.

Clock stretcher 178 also receives an IN-FRAME signal from a control, notshown, and the RESET signal. The clock stretcher 178 generates the ShiftClock which is a conditioned version of the transmitter clock. The clockis conditioned by clock stretcher 178 in two ways: First, its duty cyclecan be modified, as just described, to provide for ZERO bit insertion.Second, the shift clock is gated ON and OFF--selecting when data is tobe shifted out of the Parallel to Serial Shift Register 110 and the CRCGenerator 120. The Shift Clock is gated ON when the first byte of a newpacket is loaded into the Parallel to Serial Shift Register 110 from theFIFO buffer 150. (This load is automatic and generates a control signalto the Zero Bit Insertion Unit 124. The clock remains ON until the lastbit of the FCS is sent as indicated by the 2:1 MUX 116 control signalgoing high, or if CRC generation is not enabled (bit 5 of the DLCCommand/Control Register), until the last bit of data has sent (thepoint at which the data/FCS MUX control would have gone low if CRCgeneration had been enabled. A control line (not shown) which indicatesthe ON or OFF state of the Shift Clock is provided to the Parallel toSerial Shift Register 110 for use by its load counter 110a.

The signal generated by AND gate 182 represents the output of the ZERObit insertion unit 124 and is conducted to 2:1 MUX 130 that selectsbetween the transmission of data of Flags/Aborts. The control signal forMUX 130 is generated by the Flag/Abort generator 134. The control signalcauses 2:1 MUX 130 to select packet data transmission generated by theZero Bit Insertion Unit 124 from after the last bit of the Opening Flaguntil the first bit of the Closing Flag. The multiplexer 130 selects thesignals generated by Flag/Abort generator 134 whenever data transmissionis not explicitly selected.

The Flag/Abort Insertion Unit 134 shown in FIG. 7B generates Flag andAbort characters inserted by 2:1 MUX 130 into the data stream. The unitconsists of a shift register 186 which is loaded in parallel with eitherthe Flag (01111110) or Abort (01111111) characters stored in registers188 and 190, respectively, a divide by eight counter and the logic loadshift register 192, and the control logic 194 generating the controlsignal to the 2 to 1 Multiplexer 130.

The shift register 186 is connected to registers 188 and 190 and can beloaded with either a Flag or an Abort character. The contents ofregisters 186 are shifted out least-significant bit first by the fallingedge of the transmit clock and serially conducted to one input of 2:1Mux 130 via signal line 136.

The shift register 186 is loaded just prior to the transmission of aFlag or an Abort by unit 134. Flags are transmitted back to backwhenever the transmitter 102 is not sending either a packet (exclusiveof Opening and Closing Flags) or an Abort. Aborts are transmittedwhenever the Send Abort bit is set (bit 0 in the DLC Command/ControlRegister (App. A)). This bit is set and cleared by software. One Abortcharacter is also sent in response to a Transmit FIFO Underruncondition.

When an Abort is requested it is transmitted immediately. If the SendAbort bit is cleared in the middle of the transmission of an Abortcharacter, the transmission of the Abort character will be completedbefore the transmission of Flag characters will commence. (Flags or MarkIdle always follow an Abort. Since the Mark Idle is inserted after the 2to 1 Output Multiplexor, the Flag/Abort Insertion Unit 134 will alwaysdefault to sending Flags. If Mark Idle is Selected (bit 3 of theCommand/Control Register), the output of the DLC 52 will be forced to anall ONEs pattern, ignoring the Flags coming from the Flag/AbortInsertion Unit 134.)

When continuous Flags or Aborts are being transmitted, the shiftregister 186 is automatically reloaded after the transmission of thefirst Flag/Abort. This takes place every eight bit times. Adivide-by-eight counter within block 192 is used to divide down thetransmit clock for this purpose.

The Flag/Abort Insertion Unit 134 contains a three-bit counter whichcontains a user-supplied value indicating the number of residue bits ina packet. This value is transferred from a Residual Bit Control/StatusRegister (App. A) when the last INFO byte of a packet just prior to theFCS. The counter is decremented as each bit is shifted out of register186. When the contents of the counter reaches zero all residual bitshave been transmitted and the FCS, if enabled, and closing flag can betransmitted.

The Flag/Abort Insertion Unit 134 generates a control signal to the 2:1Multiplexer 130, indicating whether packet data or Flags/Aborts are tobe transmitted. The control signal selects the Flag/Abort path wheneverthe transmitter 102 is not sending packet data. Packet data is sent fromthe end of the Opening Flag until the beginning of the Closing Flag. Arequest to send an Abort (Send Abort bit or FIFO Underrun) places thetransmitter 102 Out of Frame.

The Serial Bus Port (SBP) 104 receives the signal selected by 2:1 MUX130. The SBP 104 performs several functions related to time slotassignment, clock selection, data inversion, enabling the transmitter,and Loop Back testing. FIG. 8 is a block diagram of the SBP 104.

The signal selected by Multiplexer 130 is routed through a Time SlotMultiplexer (TSM) where it is assigned one of thirty-one time slots, ortransmitted as is (referred to as the non-multiplexed mode). The SBP 104is designed to connect directly to the SBP 104 of the DSC 12 (FIG. 1).Up to thirty-one time slots combine to form a frame, where data istransmitted during one of the 8-bit groups shown in FIG. 9.

A transmit clock control 198 receives a Serial Frame Synch (SFS) signaland a Serial Clock (SCLK) signal applied to pins of the IDPC 10.

The Serial Frame Sync (SFS) signal (the SFS/XMITCLK pin serves as eitherthe SFS input in multiplexed mode, or the transmit clock input innon-multiplexed mode) provides a reference indicating the location ofthe first eight bits of the frame. The transmitter time-slot multiplexer196 can be programmed by microprocessor 18 (shown in FIG. 8 by thesignal line "TIME SLOT SELECTION") to place data on any one of the timeslots via bits 1 to 5 of the SBP Control Register, as described in App.A hereinafter. In the multiplexed mode a signal applied on SCLK pinprovides the transmit clock source. This clock source is gated bytransmit clock control 198 with the selected time slot to provide thetransmit clock. If time slot 0 is selected, data is transmitted for aslong as the SFS signal is active, instead of for eight bits at a time.If the SFS input is held active for 16 bit times instead of 8 eachframe, the transmitter 102 will send out 16 bits per frame, as opposedto 8. By doing this the DSC 12 can place the data on both of the two Bchannels (on an every-other-byte basis) effectively doubling the datarate. In the non-multiplexed mode (bits 1 to 5 set to ONEs in the SBPControl Register), data is transmitted continuously. In this modetransmit clock is input on the SFS/XMITCLK pin. Data is alwaystransmitted on the falling edge of the transmit clock.

After data has passed through the TSM 196 it is conducted to aprogrammable inverter XOR gate 200. If bit 0 of the SBP Control Registeris set to 1, the data will be inverted by virtue of an INVERT DATAsignal conducted to XOR gate 200 from Status/Control Registers 112.

Whenever the transmitter 102 is enabled (bit 1 of the DLCCommand/Control Register) and is out of frame (and the Closing Flag orAbort has been sent) with Mark Idle selected (bit 3 of the DLCCommand/Control Register), the transmitter's 102 output will be forcedHIGH by an XOR gate 202 receiving the signal generated by theprogrammable data inverter 200 and a MARK IDLE signal generated byStatus/Control Register block 112.

The transmitter 102 is enabled and disabled via bit 1 in the DLCCommand/Control Register (App. A). Whenever the transmitter is disabled,the SBOUT pin is three-stated by virtue of transistor 204 receiving thesignal generated by programmable Mark Idle inserter 202, uponapplication of a transmit enable (XMIT ENABLE) signal. The signalconducted via transistor 204 is generated at a Serial Bus Output (SBOUT)terminal pin of the IDPC 10.

The DLC 52 can be placed in a Local Loop Back configuration for testpurposes. This is done by setting bit 3 to a 1 in the SBP ControlRegister. Local Loop Back disconnects the SBIN and SBOUT pins (SBOUT isthree-stated) and connects the trasmitter 102 output and receiver inputtogether. The selected transmitter clock dscribed earlier is used as thereceive clock.

The DLC 52 can be placed in a Remote Loop Back configuration for testpurposes. This is done by setting bit 4 to a 1 of the SBP ControlRegister. Remote Loop Back disables the transmitter, and echoes whateveris received at the SBIN pin out the SBOUT pin. Reference should be hadto App. A for a complete description of these aspects of the IDPC 10.

Associated with the DLC 52 transmitter 102 are shown several uservisible status and control registers. These registers, contained infunctional block 112 are described in detail in Appendix A, are used toconfigure the DLC transmitter 102, instigate specific actions, reportstatus, and generate interrupts. All of these registers can be accessedby the local microprocessor 18. None of them can be accessed by a hostprocessor.

Although not shown on FIG. 4, various control and status signal linesinterconnect the DLC transmitter 102 elements shown there with theStatus and Control Register block 112. As these signal lines areconventional, and their ability to control the elements to which theyare connected is well understood by those skilled in the art.Accordingly, the design and construction of block 112, nor of the othersimilar control and status blocks employed by the IDPC 10, will beelaborated herein. Referring again to FIG. 3, the receiver sections 106and 108 of the DLC 52 takes serial data from the Serial Bus Port (SBP)104, processes it, and allows it to be moved to off-chip memory 22.Dedicated hardware modules are used to perform the bit level operationson each frame of data as it is received (Mark Idle detection, datainversion, Flag/Abort recognition, Zero Bit Deletion, CRC checking, andaddress recognition). The 16-Bit deep receive FIFO 106 is used as abuffer between this bit-rate-dependent processing and packet-at-a-timeprocessing performed by the microprocessor 18. Data can be moved fromthe receive FIFO 106 to memory 22 either by DMA, or by microprocessor 18control.

With reference to FIG. 10, a block diagram of the receiver sections 106and 108 of the DLC 52, shows the 16-byte RECV FIFO 106 interconnectingthe internal busses 58, 60 and 62 and three shift registers 208, 209 and210. Status and control registers 212 within DLC receiver 108 areconnected to busses 58, 60 and 62. Status and Control signals areconducted to and from the various elements of the receiver 108 and thestatus and control registers 212 on lines not shown in FIG. 10.

The Serial Bus Port 104 receives data signals from a Serial Bus Input(SBIN) terminal which are conducted to a shift register 212. A FlagDetect, Abort Detect Unit 214 is connected to shift register 212. Theshift register 212 is connected via a signal line 216 to the shiftregister 210 for transference of data signals.

A recovered DATA INPUT CLOCK signal generated by SBP 104 is conducted toa Bit Delete, Byte Count, Short Frame Error Unit 218 via a signal line220, which also receives the data signals on line 216. Shift register212 also receives the recovered DATA INPUT CLOCK signal on line 220. Acyclic redundancy code (CRC) Checker 222 receives the data signals online 216 as well as a clock generated by Bit Delete, Byte Count, ShortFrame Error Unit 218 via a signal line 224. Shift register 210 alsoreceives the clock signal on line 224. An address detection unit 226 isconnected to registers 208 and 210.

The major elements of the receiver sections 106 and 108, shown in blockform in FIG. 10, will be described in greater detail in conjunction withFIGS. 11-18. With reference to FIG. 11, the hardware blocks of thereceiver 108 will be discussed in the order that data flows through theunit, from the receiver portion Serial Bus Port 104 to the RECV FIFO 106(refer to FIG. 10). The receiver 102 must support data rates from DC to2.048 MHz. For this reason care has been taken in the design of thereceiver 108 to minimize the effects of real time events on the softwarethat processes the received data packets. The DLC 52 of the instantinvention has been designed to receive an entire packet and move it tooff chip memory 22 without the intervention of the microprocessor 18, ifdirect memory access (DMA) is used. Packet status information isreported on a packet-by-packet basis at the time that the packet hasbeen completely moved to memory 22. A description of this delayed statusreporting mechanism is given hereinafter in connection with FIG. 27.

The receiver portion of Serial Bus Port (SBP) 104 receives serial datafrom the SBIN pin of the IDPC 10 and generates signals therefrom to theFlag/Abort Detection Unit 214 and the Zero Bit Deletion Unit 218. Thereceive-side of SBP performs three operations on the data: Mark Idledetection, programmable data inversion, and time slot demultiplexing.FIG. 11 is a block diagram of the receive-side portion of SBP 104. Datais clocked into the receive side SBP by the rising edge of a serialclock (SCLK) signal applied at a terminal pin of IDPC 10. This signal isconducted to a clock input terminal (CLK) of a Mark Idle detector 230and to a clock input terminal of a time slot demultiplexer (TSD) 232. Atransistor 234 interconnects the SBIN terminal and a signal line 236connected to a count enable (CNT ENABLE) input terminal of Mark IdleDetector 230. Transistor 234 receives a RECEIVER ENABLE signal whichcauses the data signals applied at the SBIN terminal to be conducted onsignal line 236. The RECEIVER ENABLE signal is generated by status andcontrol register 212 as described in App. A.

Mark Idle detector 230 also receives at a complemented input an IN-FRAMEsignal, and at a complemented input the data signals conducted on line236.

The Mark Idle detector 230 examines the data stream received via line236 for the presence of 15 or more contiguous ONE bits, whenever thereceiver 108 is out of frame. The detection of Mark Idle must take placeafter the receiver 108 is out of frame since a valid data pattern of 15or more ZEROs, while In-frame on an inverted data link (inverted to allONEs), will simulate the Mark Idle condition. The Mark Idle detectionunit 230 consists of a counter that is enabled by the In-frame signalbeing inactive; counts each ONE bit as it arrives; is reset by each ZERObit that arrives; generates a Mark Idle indication signal when it counts15 ONEs and maintains that indication until a ZERO is received; iscleared by a hardware or software Reset; and is cleared and disabledwhen the receiver 108 goes In-frame upon reception of the In-framesignal.

The detection of a Mark Idle condition sets bit 0 in a Receive LinkStatus Register (App. A). If enabled, an interrupt is generated inresponse to a negative-to-positive transition of this bit.

A Programmable Data Inverter comprising XOR gate 238 receives the datasignals via signal line 236 and an INVERT DATA signal; the reception ofwhich causes inversion of the received data on a bit-by-bit basis.INVERT DATA is also generated by status and Control Register 212 asdescribed in App. A. Setting bit 0 in a SBP Control Register (App. A)causes this inversion.

Time slot Demultiplexer 232 receives the signal generated at the outputof XOR gate 238 and a Serial Frame Synchronizer (SFS) signal. The TimeSlot Demultiplexer (TSD) 232 can operate in one of two modes:multiplexed or non-multiplexed. Reference should now be had to FIG. 12,a timing diagram of the TSD 232. When in the multiplexed mode (selectedby bits 1 through 5 of the SBP Control Register (App. A) the incomingdata is valid during one of up to 31 eight-bit long time slots of a 24bit long frame. The Status/Control Register block 212 generates a signaldenoted "CHANNEL SELECT" received by TSD 232 based on bits 1 to 5 of theSBP Control Register as described in App. A. The Serial FrameSync/Transmit Clock (SFS/XMITCLK) pin of the IDPC 10 receives the framesync pulse (SFS) which is active for the first eight bit times of theframe and this defines the frame boundaries. The active time slot isselected by bits 1 to 5 of the SBP Control Register. Time slot 0 istreated as a special case in which data can be received more than eightbits at a time. When time slot 0 is selected data is received for aslong as SFS is active. This allows, for example, 16 bits of data to bereceived each frame. If the DSC 12 were using both B channels to receivedata belonging to the same packet (thus doubling the data rate), itwould send the data to the IDPC 10 over both channels 0 and 1 of its SBP104. With the SFS pulse lengthened to 16 bit times, the IDPC 10 wouldreceive all 16 bits (on its channel 0) as part of the same packet.

In the non-multiplexed mode, data is received by the TSD 232 as acontinuous stream, clocked by SCLK. Non-multiplexed operation isselected by setting bit 1 to 5 of the SBP Control Register (Refer toApp. A). In this mode, the SFS/XMITCLK input is not used by the receiver108 (it is used as the transmit clock input by the transmitter, thusgiving separate receive and transmit clocks).

The selected data signals generated by TSD 232 are generated at a DATAoutput terminal of TSD 232 and the clock used, either SFS or SCLK, isgenerated at a RECEIVE CLOCK output terminal.

With reference to FIG. 13, the Flag/Abort Detection unit 214 includes aneight-bit shift register 240 connected to the Data Output Terminal ofTSD 232. The serial receive data is shifted on the rising edge of SCLKreceived by register 240. The contents of the shift register 240 aretested by comparators 242 and 244, respectively, for the presence ofeither a Flag or an Abort character. Testing takes place every time abit is shifted into the shift register 240. In the case of Abortdetection, only the first seven bits are tested. Comparators 242 and 244generate a signal on lines 246 and 248, respectively, indicatingdetection of the Flag or Abort character, respectively. Flag/AbortDetection Unit 214 also includes comparators 242 and 244 connected toshift register 240.

The Flag/Abort Detection Unit 214 generates an IN-FRAME signal receivedby the receive and transmit sides of SBP 104 of the DLC 52, and by theShort Frame Byte Counter 260. The IN-Frame signal is generated when aFlag character is present in the shift register 240, eight bit timeselapse, and neither a Flag nor an Abort character is then present in theshift register 240.

The contents of shift register 240 are conducted on line 216 to theZero-Bit Deletion Unit 218. A divide-by-eight counter 250 is used togenerate a byte boundary signal. The counter 250 receives the SCLKsignal which it divides by eight, to produce the BYTE BOUNDARY signal.The counter 250 is reset by the receipt of a Flag detection signalconducted on line 246.

On Reset, the shift register 240 is set to all ZEROs in order to avoidfalse Flag or Abort detection.

In order to prevent valid data patterns from being detected as eitherFlags or Aborts, a technique called Bit Stuffing is used. Thetransmitter 102 examines the data stream between the Opening and ClosingFlags (exclusive). If five consecutive ONE bits are detected, a ZERO isinserted after the fifth ONE. The Zero Bit Deletion Unit in the receiver108 removes this added ZERO. FIG. 14 shows a block diagram of the ZeroBit Deletion Unit.

With reference to FIG. 14, the Zero Bit Deletion Unit 252 of Zero BitDeletion Byte Count and Short Frame Error Unit 218 is shown. The datareceived from the shift register 240 is conducted via signal line 216 tothe Zero Bit Deletion Unit 252. A three-bit counter 254 receives thedata on line 216 as well as the RECEIVE CLOCK generated by SBP 104.Signal Line 216 is connected to a count enable (CNT ENAB) input terminalof Counter 254 as well as to a complemented clear (CLR) input terminalthereof. A COUNT NOT EQUAL TO 5 (CNT 5) signal is generated at an outputof counter 254 which is conducted to an AND gate 256 as well as to acomplemented clear (CLR) input of the counter. AND gate 256 alsoreceives the RECEIVE CLOCK signal and receives at a complement input theRESET signal.

Counter 254 generates the CNT=5 signal at the presence of fiveconsecutive ONEs in the data received on line 216. If this event occurs,the next bit is deleted from the data stream (normally ZERO). Thedeletion is performed by stretching the Receive Clock one clock cycle ina Receive Shift Clock signal generated by AND gate 256. The ReceiveShift Clock is received by divide-by-eight counter 258 which alsoreceives the Flag signal on line 246 and generates a Receive CharacterClock (synchronized to Flag character receipt). The Receive Shift Clockand the Receive Character Clock signals are conducted to the Serial toParallel Shift Register 210, a Receive Byte counter portion of unit 218and the CRC Checker 222 via signal lines 224.

With reference to FIG. 15, the Short Frame Byte Counter 260 (SFBC)portion of Unit 218 includes a four-bit down counter 262 receiving at aclock input the RECEIVE SHIFT CLOCK signal that counts the number ofcharacters that have reached the Serial to Parallel Shift Register 210.Four-bit down counter 262 also includes a register 264 storing a MinimumPacket Size value received on internal busses 58, 60 and 62. Thecontents of register 264 is conducted to down counter 262 where it isapplied to a load terminal thereof. The counter 262 generates a COUNTNOT EQUAL TO ZERO (CNT=0) signal conducted to an AND gate 266. AND gate266 also receives a RECEIVE BYTE COUNT GREATER THAN ZERO (RECV BYTECNT=0) signal generated by receive 16-byte FIFO 106 and an IN-FRAMEsignal. If a frame ends in a Flag, AND the number of bytes received isless than the value programmed in the Minimum Packet Size Register, ANDdata has been placed in the FIFO (Receive Byte Counter=0), a Short FrameError signal is generated by AND gate 266.

The CRC Checker 222 is virtually identical to the CRC Generator 120 inthe transmitter and accordingly will not be further described.

FIG. 16 shows the interconnection of the Serial To Parallel ShiftRegisters 208, 209 and 210, the RECV FIFO 106, and the Address DetectionUnit 226. The data stream, as modified by the Zero Bit Deletion Unit252, is conducted to a 24-bit shift register comprising the 8-bitregisters 208, 209 and 210 which converts the serial data stream into8-bit bytes. The 16-bit contents of the shift registers 209 and 210 arepresented in parallel to the Address Detection Unit 226 for comparison.For one byte addresses, only the first eight bits of the shift register(register 210) are compared. The contents of the shift registers areconducted in parallel to the Receive FIFO 106 a byte at a time. A LOADCONTROL portion 268 of Serial to Parallel Shift Register 208, 209 and210 generates the LOAD control signal that writes the data into the FIFObuffer 106.

Serial data is received on line 216 clocked into the shift registers208, 209 and 210 on the rising edge of the Receive Shift Clock. TheReceive shift clock is active only when data is to be shifted into theshift register. The shift registers 208, 209 and 210 have three outputpaths to the RECV FIFO 106. The first output path 270 moves the leastsignificant byte (the one received first) to the RECV FIFO 106. All butthe last bytes in a packet move over this path into the RECV FIFO 106. Asecond output path 272 and a third output path 273 are used to move thelast bytes of the packet into the FIFO 106 (including the last byte ofthe FCS if the FCS is present in the packet).

Load control 268 receives the RECV CHAR CLOCK signal generated by SBP104 and generates load signals on lines 274 and 276 conducted to shiftregisters 208, 290 and 210, and RECV FIFO buffer 106, respectively. LoadControl 268 also generates a residual bit count signal which is receivedby a residual Bit Control/Status Register (App. A). The movement of datafrom the shift registers 208, 209 and 210 to the RECV FIFO buffer 106 isenabled by the rising edge of the Receive Character Clock signalgenerated by divide-by-eight counter 258 of the Zero Bit Deletion Unit252. Assuming that the packet's address matched, or address detection isdisabled, (Reference App. A) the load takes place on the nextsynchronized edge of the clock received from microprocessor 18. If thetop byte of the RECV FIFO buffer 106 is not empty at the time the loadis to take place the load is delayed, and retried on each successivemicroprocessor 18 clock cycle. If the load does not take place prior tothe receipt of the next RECEIVE SHIFT CLOCK, an Overrun error occurs.

When a Packet terminates with a Flag, an Abort, or a Long Frame Error,the last byte of the packet is moved to the RECV FIFO 106 directly. Atthe time that the closing Flag, for example, is detected, there arestill two bytes of data in the shift registers 209 and 210 and residualbits (1 to 8), if any, are located in shift register 208,left-justified. At the time the closing flag is detected, the FCS islocated in the first 16 bits of the shift registers 209 and 210. Theresidual bits (1 to 8) are located in the last eight bits of the shiftregister 208 (left justified). When the closing flag is detected, thefollowing actions take place:

The residual bit count is saved in the Residual Bit Control/StatusRegister;

The data in the last eight bits of the shift register 208 is thenshifted until the residual bit counter reaches eight;

If the FCS is not to be placed into the RECV FIFO 106, the contents ofthe last eight bits of the shift register 208 are loaded into the FIFOvia path 273 and tagged as the last byte in the packet; and

If the FCS is to be placed in the RECV FIFO 106, the last eight bits ofthe shift register 208 are loaded into the FIFO 106 via path 273, thetwo bytes of the FCS are moved into the FIFO 106 via paths 270 and 272,and the last byte of the FCS is tagged.

Any packet that terminates with a Flag or an Abort before any data hasbeen loaded into the FIFO 106, the contents of the Byte Counter areZERO, as will be described hereinafter, and in this case no data isallowed to be placed into the RECV FIFO 106. The contents of the shiftregisters 208, 209 and 210 is simply ignored--it will be cleared out bythe first 16 bits of the next packet.

The Address Detection Unit 226 is used to identify packets that areaddressed to the receiver 108. Depending on programming viaStatus/Control Registers 212, the first one or two bytes of eachreceived packet is compared against up to five Address Registers (fouruser programmable and one broadcast). If the incoming packet's addressfield matches one of the Address Registers, if enabled, the packet isreceived. If no match occurs the packet is discarded and the receiver108 reenters looking for a Flag state. The states which the receiver 108can assume are described hereinafter in connection with FIG. 20.

The Address Detection Unit 226 is illustrated in FIG. 17 and includesfive comparison units 278, 280, 282, 284 and 286. The first fourcomparison units 278-284 include a programmable 16-bit address registerand a two-byte comparator. The last comparison unit 286 (broadcast)consists of register containing all ONEs and a two-byte comparator. Acontrol element 288 receives the RECV CHAR CLK signal and is connectedto an Address Control Register within element 212. Associated with eachcomparison unit is an enable bit that turns that particular recognitionunit on or off. These bits reside in the Address Control Register (App.A). If all five enable bits are cleared (disabled) the receiver 108 willaccept all packets. Bit 5 of the Address Control register selectswhether the address is one or two bytes long. If one-byte addressing isselected, either the least significant eight bits of the sixteen bitAddress Registers or the most-significant eight-bits, as selected by bit7 of the Address Control Register, are used in the comparison. Also, bit6 of the Address Control Register causes the second bit (bit 1) of thefirst byte of all addresses to be ignored. This is required since someBOPs use this bit position to indicate whether the packet is a commandor a response (C/R). When this ignore C/R bit control bit is set, bit 1of the first byte of all addresses is considered a don't care. Refer toApp. A for a complete description of the Address Control Register.

Address comparison takes place when the Serial to Parallel ShiftRegisters 209 and 210 have received 16 bits following the Opening Flag.The identity of the particular comparator that makes the match with theincoming address is reported in bits 0-2 of the Interrupt SourceRegister described in App. A. This status is reported when the last byteof the packet is read from the RECV FIFO 106.

With reference to FIG. 18, the Receive FIFO 106 interconnects the Serialto Parallel Shift Registers 208, 209 and 210 and the Internal Busses 58,60 and 62 and consists of a RECV FIFO buffer 290, a Receive Byte Counter292, and a 4-stage Receive Byte Counter 294 and Data Request controllogic 296.

The Receive Byte Counter 292 is a read/write register and is connectedto the internal busses 58, 60 and 62 and receives a LOAD signalgenerated by the shift register load control 268.

The 4-stage Receive Byte Counter 294 is a read-only register and isconnected to the internal busses 58, 60 and 62 and receives anEND-OF-FRAME TAG signal from a data register portion 298 of FIFO buffer290.

The END-OF-FRAME TAG signal is used by the 4-stage delayed statusreporting mechanism employed by Receive Byte Counter 294 as will beexplained hereinbelow in connection with FIG. 27.

The receive FIFO buffer 290 is a 32-byte deep buffer that is loaded atthe "top" from the Serial to Parallel Shift Registers 208 and 210 andunloaded at the bottom via the Data Register 298 by the microprocessor18 or DMA. (Refer to description of data movement given in connectionwith FIG. 16). Data is shifted down the buffer at the clock ratesupplied by the microprocessor 18.

The presence of data in the Data Register 298 is indicated by thesetting of the Data Available bit (bit 1) in FIFO Status Register (App.A) portion of Status and control registers 212. This bit is cleared whenthe Data Register 298 becomes empty and the location in FIFO buffer 290directly above the Data Register 298 is empty.

When the receiver 108 terminates the receipt of a packet (normally orabnormally), and data from that packet has been placed in the receiveFIFO buffer 290, the last byte of the packet is tagged when it is placedinto the buffer. Each FIFO buffer 290 location contains a ninth bit toaccommodate this tag. The presence of a tagged bit in the buffer 290forces Data Request, described hreinafter, active.

Associated with the receive FIFO buffer 290 is the Data Request Controllogic 296 which generates a Threshold Reached signal. The Data RequestControl logic 296 receives a signal from FIFO buffer 290 indicating thenumber of bytes stored in the buffer 290. This signal is active wheneverthe number of bytes of data in the buffer 290 is equal to or greaterthan the threshold level programmably stored in a FIFO ThresholdRegister (App. A). When Threshold Reached is active, bit 0 in a ReceiveFIFO Status Register is set to 1. A maskable interrupt is generated whenthe Threshold Reached bit transitions from ZERO to ONE. The ThresholdReached signal is also used in the generation of Data Request to theDMA.

If the "top" location in the receive FIFO buffer 290 is full when theSerial To Parallel Shift Register load control 292 generates a loadsignal for a new byte of data, an Overrun condition occurs. This erroris described in detail hereinafter.

Data Request Control logic 296 of the FIFO 190 generates a Data Requestsignal that controls the operation of the DMA (when used). Data Requestsignal being active informs the DMA that it should empty the FIFO buffer290. Data Request signal goes active when the Threshold Reached signalbecomes active, OR, a byte tagged as the end of a packet is present inthe FIFO buffer 290. Data Request remains active until the buffer 290becomes empty, OR, the tagged byte has been removed.

A 16-bit Receive Byte Counter 292 is provided in the receive FIFO 106 tomaintain a count of the number of bytes that have been placed in theFIFO buffer 290 from the packet that is currently being received. Whenthe last byte of the packet (tagged as such) is removed from the FIFObuffer 290 the contents of the Receive Byte counter 292 is conducted toa Receive Byte Count Register 294. This is a four stage register thatdelays the reporting of the byte count until the tagged byte is readfrom the FIFO 290--this delayed reporting opertion is described indetail hereinafter. The Receive Byte Counter 292 is cleared when itscontents are loaded into the Receive Byte Count Register 294.

The Receive Byte Count Register 294 reports the length of the receivepacket to software. This read only register is connected to the InternalBusses 58, 60 and 62. A description of the delayed-reporting four stageregister is given hereinafter in connection with FIG. 27.

Associated with the DLC 52 receiver 108 are several user visibleregisters. These registers, which are described in detail hereinafterare used to configure the receiver, instigate specific actions, reportstatus, and generate interrupts. All of these registers can be accessedby the local processors; none of them can be accessed by an off-chiphost processor. Appendix A contains a description of the various DLC 52registers.

Briefly, normal operation of the transmit-side IDPC 10 will be describedby reference to FIG. 19, a transmit-side state diagram. Followinghardware Reset of the IDPC 10, or when the IDPC 10 is reset by themicroprocessor 18 (bit 6 of the DLC Command/Control Register withinblock 112), the transmitter 102 of the DLC 52 is disabled, and is instate 0a-sending Mark Idle-denoted element 300 in FIG. 19.

The microprocessor 18 initializes the DLC transmitter 102 by settingbits within transmitter status/control registers 112, described in App.A, by selecting data inversion or non-inversion (bit 0 of the Serial BusPort (SBP) Control Register, selecting the SBP channel configuration(bits 2-1 in the SBP Control Register), selecting whether CRC generationis to be used, and selecting either Flag or Mark Idle (bit 3 of the DLCCommand/Control Register, the default is Mark Idle).

The Transmit Byte Count Register 152 (App. A) is used to specify thelength of the packet to be transmitted, excluding FCS bytes, and is onlyprogrammed when the packet length to be transmitted is different fromthe previous packet transmitted. Bytes are counted in the transmit bytecounter 154 in the transmit FIFO 100 as they are placed into thetransmit FIFO buffer 150. When the count equals the value programmedinto the Transmit Byte Count register 152, that byte is tagged as thelast non-FCS byte in the packet. Data inversion/non-inversion and SBPchannel configuration do not affect the operational sequence of thetransmitter 102. The Flag Idle/Mark Idle selection does affect theoperational sequence and is described below.

After the DLC transmitter 102 is Reset (bit 6 of the DLC Command/ControlRegister, or hardware Reset), the transmitter 102 goes to state 0a 300.The transmitter will remain in state 0 until data has been placed in thetransmit FIFO 100, and the first byte of that data reaches the top ofthe FIFO. At that time the transmitter 102 will go to state 1 (element302).

With the transition to state 1 (302) the transmitter 102 is said to be"In-frame". In state 1 the transmitter 102 sends the Opening Flag. Whenthis Flag has been sent state 2 (304) is entered.

While in state 2, data is unloaded from the transmit FIFO buffer 150into the eight bit Parallel to Serial Shift Register 110. Serial data isclocked out of the shift register, through the 2 to 1 multiplexer 116,and into the Zero Bit Insertion Unit 124. The data is then fed into theSerial Bus Port (SBP) 104 where it is optionally inverted, andtransmitted to the data communications network. The transmitter 102leaves state 2 when the last byte of the packet up to the first FCS bytehas been shifted out of the Parallel to Serial Shift Register 110.

If CRC generation has been selected (bit 5 of the DLC Command/ControlRegister) the transmitter 102 will enter state 3 (306). If CRCgeneration is disabled, state 4 (308) is entered directly from state 2.In state 3 the inverted contents of the CRC generator 120 is fed to theZero Bit Insertion Unit 124 following the original packet (nowcompleted) data stream. The inversion of the CRC generator 120 output isrequired by the CRC algorithm. After the 16 bits of the FCS (invertedcontents of the CRC generator) have been transmitted the Valid PacketSent bit is set (bit 4 in the Interrupt Source Register) and State 4(308) is entered. The Valid Packet Sent indication can generate amaskable interrupt.

While in state 4 one Flag character (the Closing Flag) is transmitted.The transmitter 102 will transition to either state 0a (300) 0b (310) or1 (302) when the transmission of the Flag completes. If data is presentin the transmit FIFO 100 (a new packet), state 1 is entered. If no datais present in the FIFO 100, state 0 is entered. The selection of theFlag Idle or Mark Idle inter-frame fill (bit 3 of the DLCCommand/Control Register) selects between state 0a and 0b.

There are five exceptions to the normal flow of events described above:Abort, Local Loop Back, Remote Loop Back, transmitter disabled whileIn-frame, and FIFO Underrun. Of these only FIFO Underrun is an errorcondition.

The user can terminate the transmission of a packet by requesting thatan Abort be sent (bit 0 of the DLC Command/Control Register). When aSend Abort request is received the transmitter 102 enters state 5 (312)where the transmitter 102 will begin transmitting Abort characters(01111111 with 1 being the LSB). This action takes place on the next bitboundary after the Send Abort bit is set by software; the transmit FIFO100 will be cleared. Abort characters will continue to be sent untilthis bit is cleared. The transmitter 102 will go out of frame whentransmission of the Abort begins. When the Send Abort bit is cleared thetransmitter will enter state 0b if Flag Idle is selected or data ispresent at the top of the FIFO (a new packet); state 0a is enteredotherwise. In all cases at least one Abort character will betransmitted, even if the Send Abort bit is set and cleared byconsecutive CPU instructions. (The Abort is used to tell the receiver onthe other end of the link that the packet currently being received is tobe terminated and discarded.) While sending an Abort has no meaning whenthe transmitter 102 is out of frame (not sending a packet), the requestwill be honored. It will have no meaning at the receive end if thereceiver is out of frame.

For test purposes the DLC 52 can be placed in a Local Loop Back mode ofoperation (bit 3 of the SBP Control Register (App. A). In this mode theDLC transmitter 102 is disabled at the same point that the TransmitEnable bit (bit 1 of the Command/Control Register is implemented. TheDLC receiver 108 is also disabled to prevent incoming data frominterfering with the Loop Back. The transmitter 102 is then connected tothe receiver 108 and the transmit clock is used as the timing referencefor both the transmitter 102 and the receiver 108. Packets can then betransmitted normally with the exception that nothing leaves the part.The receiver 108 receives the packet just as if it were originating fromoutside the IDPC 10. See App. A for loop-back operation.

Remote Loop Back, selected by setting bit 4 of the SBP Control Register,causes any activity on the SBIN input to the receiver 108 to be echoedon the SBOUT output pin. The DLC transmitter 102 is disconnected fromthe SBOUT pin via transistor 204. When the SBP is operating inmultiplexed channel mode each received bit (conditioned by SFS/XMITCLK)is transmitted on the next falling edge of the receive clock i.e., datareceived at the SBIN pin on the rising edge of SCLK is clocked out ofthe SBOUT pin by the subsequent falling edge of SCLK. When the SBP 104is operating in the non-multiplexed mode, data bits received via SBIN(clocked in by the positive going edge of the receiver clock [SCLK]) isclocked out on a bit by bit basis using the negative edge of the sameclock (SCLK). The DLC receiver 108 can still receive data while in thisstate.

If an attempt is made to use the DLC transmitter 102 102 while in RemoteLoop Back mode, the transmitter will function normally, but no data willleave the IDPC 10.

DLC Transmitter 102 is disabled while In-Frame. The transmitter willcontinue to process the frame normally, and will disable the SBOUT pinas soon as the Closing Flag has been sent. Once the Closing Flag istransmitted, the transmitter 102 returns to state 0 and disconnects theSBOUT pin via transistor 204 (places it in an open drain condition withno ability to be driven low).

A FIFO Underrun occurs when the transmitter attempts to unload a byte ofdata from an empty transmit FIFO 100 while in frame. This condition isreported via bit 4 of the FIFO Status Register (App. A) and a maskableinterrupt is generated. This causes the FIFO Status Register bit to besent in the Interrupt Source Register (App. A) (if the Underruninterrupt has been enabled in the FIFO Status Interrupt EnableRegister). When the FIFO Underrrun is detected the DLC transmitter 102enters state 6 (314) where one Abort character (01111111) is transmittedand the transmitter 102 reenters state 0.

Briefly, normal DLC receiver 108 operation will be described byreference to FIG. 20, a receive-side state diagram. Following hardwareReset of the IDPC 10 or when the bus 14 is reset by software (bit 6 ofthe DLC Command/Control Register), the DLC receiver 108 is disabled andis in a state 0 (316 in FIG. 20).

When the receiver 108 is disabled (by clearing bit 2 of the DLCCommand/Control Register), the connection between the SBIN pin and thereceiver is severed via transistor 234. This is the only effect thatdisabling the receiver 108 has on the remainder of the DLC 52. All otherreceiver functions work in the same manner as they do when the receiver108 is on.

The user, via software running on the microprocessor 18, initializes theDLC receiver 108 by: selecting data inversion/non-inversion (bit 0 ofthe SBP Control Register (App. A)), specifying SBP channel configuration(bits 1 and 2, of the SBP Control Register), enabling CRC check ifdesired (bit 4 in the DLC Command/Control Register (App. A)), selectingthe desired address mode (Address Control Register (App. A)), loadingthe address(es) to be recognized (Address Register(s)), specifying theminimum packet size Minimum Packet Size Register 264 (App. A)),specifying the maximum packet size (Maximum Packet Size Register (App.A)), and finally enabling the receiver 108 (bit 2 in the DLCCommand/Control Register.

The DLC receiver 108 starts operation in state 0 (316). In state 0 thereceiver 108 examines the incoming data stream (clocked in from the SBINpin on the rising edge of SCLK {SCLK pin]) on a bit by bit basis for thepresence of a Flag character. No data is passed beyond the Flag/Abortdetection unit 214 in state 0. The detection of a Flag causes atransition to a state 1 (318).

In state 1 the data stream is inspected on a character by characterbasis for the presence of a non-Flag, non-Abort character (characterboundaries are established by the receipt of a Flag). If the characterfollowing the Flag is another Flag, the receiver remains in state 1. Ifthe character is an Abort, the receiver reenters state 0. If thecharacter is not a Flag or an abort, the receiver 108 is said to beIn-frame, and a state 2 (320) is entered.

In state 2 data is passed beyond the Flag/Abort detector 214 to the ZeroBit Deletion Unit 218. Here, the next bit following any five contiguousONEs is deleted (this bit should always be a ZERO, and was inserted bythe transmitter to prevent data patterns from being detected as Flag orAbort characters--which have six and seven contiguous ONE bitsrespectively). The first one or two characters following the OpeningFlag of the packet are normally the address field (while the addressfield can be more than two bytes long, the receiver 108 can examine onlythe first two bytes of any address--any remaining bytes are treated asdata). If address recognition is enabled (bits 0-4 of the AddressControl Register), these characters are tested by the address detectionunit 226 for a match with one of the five enabled preprogrammedaddresses (four programmable addresses and the Broadcast Address). Ifthere is not a match, the DLC receiver 108 returns to state 0 (lookingfor Flags). The packet currently being transmitted is ignored and nostatus is reported on it. If, however, there was an address match (oraddress detection was disabled, in which case all frames are accepted),the frame is received and is placed into the receive FIFO 106, one byteat a time (including the Address, Control, Information, and FCS fields).Each received character is loaded into the receive FIFO 106 108 when itreaches the last eight bits of the 16 bit long Serial to Parallel ShiftRegister 208, 210 (with the exception of the last character, discussedbelow).

State 2 is exited normally whenever the Flag/Abort detector 214 receivesa Flag character. If a Flag is detected the receiver 108 enters state 1.(Back to back packets can share Opening and Closing Flags.) At the timethe Flag is detected, the two previous characters still in the 16 bitlong Serial-To-Parallel Shift Register 208, 210 are immediately loadedinto the receive FIFO 106 and the second of these two bytes is tagged asthe last character in the packet. The tag makes the form of a ninth bitappended to each word in the FIFO 106. If CRC checking has been enabled(the output of the CRC comparator 222 is valid at this time), and itsstatus (error or not) is recorded. These last two characters loaded intothe receive FIFO 106 are the Frame Check Sequence (FCS) if CRC check isenabled.

When a packet has been received with either a Closing Flag, an Abort, ora Long Frame Error, its length and status are latched. This informationis presented to the user when the last byte of the packet (tagged assuch) is read from the receive FIFO 106 (by DMA or programmed I/O). Aninterrupt (maskable) indicating the receipt of a packet, and its status,are generated at this time. The delay in status reporting is requiredsince the user's software operates at a packet level and has notreceived the complete packet until the last byte has been moved from thereceive FIFO 106 to memory 22. In normal operation the receive FIFO 106is automatically unloaded by the DMA and the user is not interested inthe status of a packet until it has been completely transferred tomemory 22.

During the course of normal operation six error or exception conditionscan occur. These are: the receipt of an Abort character while In-frame,a CRC Error, a Short Frame Error, a Long Frame Error, a Non-IntegerNumber of Bytes Error and a FIFO Overrun Error. In addition to these sixcases, the DLC receiver 108 can be placed in two test modes: Local LoopBack and Remote Loop Back.

When an Abort is received while the receiver 108 is In-frame (state 2),the packet is terminated. The Abort takes precedence over all receiveerrors. As a result of this termination several actions are taken: thecontents of the 16 bit shift register 208, 209, 210 is moved to thereceive FIFO 106. The last byte is tagged as such as it is placed intothe FIFO; the DLC receiver 108 returns to state 0; The status, includingthe Abort Received bit in the Receive Link Status Register (App. A) andbyte count are latched; and when the last byte of the aborted packet isread from the receive FIFO 106, a maskable interrupt is generated.

When the Closing Flag of a packet is detected, the CRC Checker 222 hasfinished its work. If CRC checking is enabled (bit 4 in the DLCCommand/Control Register), the output of the CRC checker 222 is testedat this time. If an error has occurred, this error condition is latchedfor delayed reporting.

When a packet is terminated (with a Flag) that has fewer characters(exclusive of Flags) than is programmed into the Minimum Receive PacketSize Register, and more than 16 bits, a Short Frame Error is reported.If the packet had 16 or fewer bits it is discarded without notificationto the user. This is possible since no data has been placed into thereceive FIFO 106 at this time. If the Short Frame contained more than 16bits, it is terminated the same way that a normal packet is, with theexception that the Short Frame Error is latched for delayed reporting.The receiver 108 returns to state 1.

The DLC receiver 108 contains a Maximum Receive Packet Size Register(App. A) within block 212 which is programmed to specify the maximumacceptable packet length. If the number of bytes received equals thiscount and a Flag or an Abort is not detected at this time, a Long FrameError exists and the packet is terminated. This termination is the sameas for a normal frame with the exception that the Long Frame Errorstatus condition is latched for delayed reporting.

If a flag is detected on a non-byte boundary (when from 1 to 7 bits of acharacter have been received), a Non-Integer Number of Bytes Errorexists. The packet is terminated as normal with the exception that theshort character is loaded into the receive FIFO 106 as is (it is taggedas the last byte), and the Non-Integer Number of Bytes Error status islatched for delayed reporting Note that in an alternative embodiment ofthe IDPC 10, the DLC 52 can receive and transmit packets containing anon-integral number of bytes.

When a byte has been shifted into the last 8 bit positions of the 16 bitshift register 208, 210 it is moved into the receive FIFO 106. There isone bit time for this operation to take place. If the top location inthe receive FIFO 106 is full when this load is attempted, the load isblocked. If the top location in the FIFO 106 does not become emptybefore the next bit is to be shifted into the shift register, a FIFOOverrun condition exists. When this happens the packet is terminated,the last byte in the FIFO is tagged as the last byte in the packet,status is latched--including the Overrun condition indicator--fordelayed reporting, and the receive returns to state 0 (if a Flag isdetected at the same time as the Overrun then state 1 is entered).

For test purposes the output of the DLC transmitter 102 can be LoopedBack to the receiver 108. This mode is selected by setting bit 3 in theSBP Control Register (App. A). When in the Local Loop Back mode, thereceiver is isolated from its input (SBIN pin) via transistor 234.

For test purposes the input of the DLC receiver 108 can be fed directlyto the output pin of the transmitter 108 (SBOUT). This mode is enteredwhen bit 4 of the SBP Control Register is set. The operation of thereceiver 108 is unaffected by this action.

FIG. 21 is a functional block diagram of the UART 54 portion of the IDPC10 of the present invention. The UART 54 of the instant invention iscompatible with the industry standard 8250 UART as described at pages357-371 of "Communications Products Handbook" published by WesternDigital corporation, copyright 1984, which description is herebyincorporated herein, and, in addition, provides operation in asynchronous mode as well as an asynchronous mode. The UART 54 alsoincludes a Special Character Recognition Unit and transmit and receivefirst-in, first-out (FIFO) registers.

As shown in FIG. 21, UART 54 includes a 10-bit receiveserial-to-parallel shift register 400 connected via a signal line to aReceive Data input terminal of the IDPC 10. Receive shift Register 400is connected to a Receive FIFO 404 via a bus 406. Data signals arereceived by register 400 and transferred to FIFO 404 under control ofsignals received from a UART control 408 which includes a set of Statusand Control Registers. Receive Shift Register 400 and Receive FIFO 404are connected to UART Control 408. Receive FIFO 404 contains a receiveFIFO data register 404a located at the "top" of receive FIFO 404, i.e.,that portion of FIFO 404 connected to busses 58, 60 and 62.

A Parity, Special Character, Frame, Break Checker 412 is connected toReceive Shift Register 400, Receive FIFO 404 and to UART control 408.Parity, Special Character, Frame, Break Checker 412 includes a randomaccess memory 413, as will be described in greater detail hereinafter inconnection with FIG. 22.

A receive multiplexer 410 connected UART control 408 receives ReceiveClock signals on a signal line 409 connected to an input terminal of theIDPC 10. The receive clock MUX 410 has an output terminal connected toReceive Shift Register 400. A Baud Clock signal generated by a Baud RateGeneration Unit 414 is conducted on a signal line 416 to a second inputterminal of receive Clock MUX 410, as well as to a first input of atransmit clock MUX 418, also receiving the Receive Clock via signal line409 and connected to UART control 408. The output terminal of transmitclock MUX 418 is connected to a transmit (XMIT) parallel-to-serial shiftregister 420.

The XMIT shift register 420 receives signals via a bus 422 from atransmit (XMIT) FIFO 424 which, in turn, is connected to the busses 58,60 and 62. The XMIT shift register 420 is connected to UART control 408,as is the XMIT FIFO 424. The XMIT parallel-to-serial shift registergenerates signals conducted on a signal line 426 to a XMIT Data terminalof the IDPC 10.

Transmit FIFO 424 contains a transmit FIFO data register 424a located atthe bottom of Transmit FIFO 424, i.e., that portion of FIFO 424connected to busses 58, 60 and 62.

A Parity, Frame, Break Generation Unit 428 is connected to UART control408 and to XMIT shift register 420 and XMIT FIFO 424.

A UART Interrupt controller 430 is connected to busses 50, 60 and 62 andgenerates signals to UART control 408 via a signal line 432. UARTcontrol 408 receives clear to send (CTS/), data set ready (DSR/), ringindicator (RI/), and received line signal detect (RLSD/) signalsconducted from an input terminal of the IDPC 10 and generates return tosend (RTS) and data terminal ready (DTR) signals conducted to an outputterminal of the IDPC 10.

The clocks used to transmit and receive data by UART 54 come from one oftwo sources: the receive clock (RXCLK) input in, or the baud rategenerator 414. Clock selection is made via bits 0 and 1 in a UARTControl Register within Status and Control Register block 408. AppendixB contains a complete description of the registers within UART block408. As with the DLC Status/Control Registers, various status andcontrol signals are conducted between these registers and the UARTelements shown in FIG. 21, not shown.

The baud rate generator 414 is a programmable divider that receives asignal via line 414 applied to the UARTCLK pin. Generator 414 provides aBaud clock, to the UART receiver and transmitter sections. The inputclock received on signal line 414 is divided down by a programmable 16bit (1-65536) Divider. The programmable divider is configured by loadingthe Divisor Latch LSB and Divisor Latch MSB Registers (App. B). Theseregisters are accessed by setting the Divisor Latch Access Bit (DLAB),bit 7 in the Line Control Register (App. B) of control 408, and thenwriting the UART addresses 0 and 1 (these are the DATA Registers andInterrupt Enable Register addresses when the DLAB bit is cleared).

In the asynchronous mode the baud rate generator 414 is similarlyprogrammed to a value 16 times the receive clock rate.

The sources of the transmitter and receiver clocks are independentlyselectable via receive clock MUX 410 and transmit clock MUX 418,respectively. For example: When bit 0 is set in the UART ControlRegister, the receiver clock MUX 410 selects the output of the baud rategenerator 414 for its clock. When bit 0 is cleared, the RXCLK input isused. The same options apply for the transmitter clock MUX 418 exceptthat in this case bit 1 in the UART Control Register (App. B) specifiesthe clock source.

The UART 54 has two primary modes of operation: asynchronous andsynchronous.

In the asynchronous mode the receive and transmit shift registers 400and 420 are clocked at a rate that is 16 times the baud rate.Asynchronous operation is selectable via microprocessor 18 by clearingbit 2 of the UART Control Register (App. B) of Control 408 to ZERO. Asdescribed above, the source of the clock can be either the internal baudrate generator 414 or an external input (receive clock input, RXCLK).Receive clock selection is determined by bit 0 of the UART ControlRegister, transmit clock selection by bit 1 of the UART ControlRegister.

In synchronous operation the receive shift register 400 is clocked atthe same rate as the data. This means that the data and clock must be insynchronization with each other. Data is latched into the receive shiftregister 400 on the rising edge of the clock. Synchronous mode isselected by setting bit 2 of the UART Control Register.

The clock used by the transmit shift register 420 is also the data rate.Data is shifted out of the shift register 420 on the falling edge of theclock. The transmit clock can be provided by either the baud rategenerator 414 or the external receive clock input (RXCLK).

Data is transmitted as a steady stream of bits with no framing (startand stop bits) involved. When the transmit shift register 420 is loaded,its contents are transmitted directly. The next data byte isconcatenated onto the previous one. When the shift register 420 and FIFO424 are empty the line is placed in a Marking (ONEs) condition.

Data is received on line 402 as a steady stream of bits with no framinginvolved, and therefore no character boundaries. As eight bits arereceived into the received shift register 400, they are loaded into theReceive FIFO 404. When the line 402 is idle (Marking), the receive shiftregister 400 is receiving (and moving to the FIFO) bytes containing allONEs. This mode is useful in low speed synchronous applications sincethe end to end link--IDPC UART, to ISDN, to IDPC UART--appears as apiece of wire to the two end users. Data is sampled and transferred aslong as receive clock pulses are received.

The receive shift register 400 is used to convert incoming serial datainto parallel characters. The serial data is clocked into the shiftregister 400 by the data sample strobe signal on line 416 inasynchronous mode and the rising edge of the receive clock on line 412in synchronous mode as selected by clock MUX 410.

Asynchronous operation--The data sample strobe signal is generated byBaud Rate Generator 414 in the following manner: Whenever the UARTreceiver is not receiving a character, the rising edge of the 16X clockis used to sample the receive data (RXD) input signal on line 402. IfRXD has transitioned from high to low since the last rising edge of the16X clock, a potential start bit has been detected. If the RXD signalline 402 remains low for at least three clock cycles, the start bit isassumed to be valid; if not, it is ignored. Assuming the start bit wasdetermined to be valid, the eighth rising clock edge after the high tolow transition of the RXD signal (beginning of the start bit) is used tosynchronize a divide by 16 counter within Baud Rate Generator 414 thatdivides the 16X receive clock down to form the data sample strobe (BaudClock) signal. The RXD signal on line 402 is then sampled by the BaudClock once for each bit in the character to be received.

The receive shift register 400 is ten bits long permitting up to eightdata bits, one parity bit, and the start bit. When the previouscharacter is moved from the shift register 400 to the FIFO 404, or atReset, the shift register 400 is loaded with all ONEs. Data is shiftedinto the shift register start bit first (the start bit is a ZERO). Whenthe start bit reaches the end of the shift register (bit position tentransitions from a high to a low) the character has been completelyreceived. Note that for characters of less than eight bits (or an eightbit character without parity), the data is loaded into the shiftregister 400 at a bit position that is closer to the low order bitposition such that the start bit will wind up in the last bit positionat the end of the character. This technqiue eliminates the need for acounter to keep track of the number of bits received.

If the RXD signal is sampled low on the next bit time after the last bitof a character is received, a framing error exists, and is reported viabit 3 of the Line Status Register. The character with the framing erroris not loaded into the FIFO 404.

When the UART 54 receives characters containing less than eight databits, the additional high order bits in the 8 bit byte that is to beloaded into the receive FIFO ar set to ZERO.

Synchronous operation--In synchronous mode the RXD input signal issampled on every rising edge of the 1X receive clock received on signalline 409. Data is shifted into the receive shift register 400 on everyclock cycle. In this mode there are no start or stop bits. One byte ofdata is received and loaded into the receive FIFO 404 every eight bittimes.

Received data is loaded into a four byte deep receive FIFO 404. ReceiveFIFO 404 is preferably of the "bubble up" type. An interrupt conditionflag is set in the Interrupt Identification Register (bits 1-3) when thenumber of characters in the FIFO 104 has reached the level designated inthe receive FIFO 404 threshold field of the UART Control Register (bits3 and 4). Bit 3 in the UART Status Register is set when the receive FIFO404 threshold is reached, and cleared when the FIFO 404 level falls backbelow the threshold. A timeout is generated internally if the number ofcharacters received is less than the FIFO threshold level (0), and nocharacters have been received for 1600 baud clock cycles in theasynchronous mode, and 100 clock cycles in the synchronous mode(approximately ten character times). The timeout sets bit 0 in the UARTStatus Register and generates a maskable interrupt.

Data is read out of the FIFO 404, from a Receive FIFO Data Register 404aconnected to busses 58, 60 and 62 by the microprocessor 18. The presenceof valid data in the Receive FIFO Data Register 404a is indicated by(Receive Data Available) bit 0 in the Line Status Register.

If the receive FIFO 404 is full when a newly received character is to beloaded into the FIFO, an Overrun Error is reported via bit 1 in the LineStatus Register.

With reference to FIG. 22, a parity checker 412a and RAM 413 portion ofparity, special character, frame, Break Checker 412 is shown connectedto the receive FIFO 404 which is ten bits wide, eight data bits, onespecial character flag, and one parity error flag. Parity, framing, andspecial character conditions are checked by element 412 when data isloaded into the FIFO 404. In particular, the presence of a characterthat has a parity error is reported by parity checker portion 412a, oris a special character is reported by comparison with RAM 413, in theLine Status Register. The parity error flag and the special characterflag are set accordingly. Interrupts (if enabled) are generated wheneither condition is detected. Only the data bits can be read by theuser. While Special Character and Parity Error interrupts are generatedwhen the character is loaded into the receive FIFO 404, the Parity Errorpresent and Special Character Available status bits (in the UART StatusRegister) are not set until the character is at the FIFO output 404a.This allows the user to identify which character caused the interrupt.

When the UART 54 is programmed to receive characters with fewer thaneight bits, the unused bit positions are filled with ZEROs as thecharacter is placed into the receive FIFO 404.

Special Character Recognition is performed within element 412 connectedto receive shift register 400 and receive FIFO 404. When a validcharacter has been received by shift register 400, the lower seven bitsof its bit pattern are used as a pointer into the 128 bit deep RAM 413.Bits in the 128 bit RAM 413 are set and cleared by the microprocessor18. If the RAM bit addressed by the data is set (1), the character isflagged as "special" by the setting of bit 7 in the Line StatusRegister. This test is performed when the character is loaded into thereceive FIFO 404. An interrupt is generated only if the specialcharacter enable bit is set (bit 5 in the Interrupt Enable Register).The special character received bit is used to identify the presence ofan interrupting condition. A second bit, bit 2 in the UART StatusRegister, is used to identify which character in the FIFO 404 isspecial. This bit is not set until the character is at the output of theFIFO.

The special character recognition unit 412 uses the received charactersas addresses into a 128 bit RAM 4b. The 128 bit RAM 413 is organized as128×1, as seen by the special character detector, and as a 16×8 array asseen by the user. RAM 413 is loaded by the user by writing to one ormore of the 16 registers. The first register, located at relativeaddress 9, contains the first eight bits of the 128 bit map. Bit 0 inthe map corresponds to bit 0 in the first register. Bits 15-8 arelocated in the second register (address 10) and so forth. The defaultvalue at Reset is all ZEROs.

Parity is checked by parity checker element 412a on all receivedcharacters as they are loaded into the receive FIFO 404. If a violationhas occurred, and parity is enabled (bit 3 of the Line Control Register(App. B)), the parity error bit is set (bit 2 of the Line StatusRegister (App. B)). If the receiver line status interrupt is enabled(bit 2 of the Interrupt Enable Register (App. B)), an interrupt will begenerated. A second status bit, bit 1 in the UART Status Register (App.B), is set when the character containing the parity error reaches theoutput of the receive FIFO 404. This allows the user to identify whichcharacter in the FIFO 404 contains the error. The selection of even orodd parity is made via bit 4 of the Line Control Register. The UART 54can be placed in a test mode that forces the parity bit to be generatedand then detected in the state opposite the programmed state (even orodd). This stick parity mode is invoked by setting bit 5 in the LineControl Register.

Frame errors are detected by element 412. Framing is valid only in theasynchronous mode of operation. Framing is not checked in thesynchronous mode.

Bit 3 of the Line Status Register is set if the received character doesnot have a valid stop bit, and is not a break condition. An interrupt isgenerated if the line status interrupt enable bit is set (bit 2 of theInterrupt Enable Register).

Break Detection is performed within element 412. Break detection isvalid only in asychronous mode. Break detection does not take place insynchronous mode.

Bit 4 in the Line Status Register is set if the receive data input isheld spacing (0) for more than a full character time (start bit+databits+parity bit +stop bits). The Receive Line Status interrupt must beenabled for an interrupt to be generated.

Data that has been transferred into the transmit FIFO 424 by themicroprocessor 18 via busses 58, 60 and 62 is loaded into theparallel-to-serial transmit shift register 420 and shifted outsynchronized by the transmit clocked selected by transmit clock MUX 418.Parity can be generated and appended to the data. The character lengthand number of stop bits are programmable via microprocessor 18. Breakindications can be generated by the UART transmitter.

The shift register clock applied to transmit shift register 420 can comefrom either the baud rate generator 414 or the receive clock receivedfrom the input pin as selected by transmit clock MUX 418. Clock sourceselection is made via the transmit clock selection (bit q) in the UARTControl Register. The input source for the shift register clock is 16times the data rate in asynchronous mode, and 1 times the data rate insynchronous mode. Synchronous operation is selected via bit 2 of theUART Control Register). In Asynchronous Mode (only) the transmit logicautomatically divides the clock by 16. Data is shifted out of shiftregister 420 on the falling edge of the clock.

Bit 6 of the Line Status Register is set when the transmit FIFO 424 isempty and the last bit has been shifted out of the shift register 420.An interrupt can be generated by this condition.

Data to be transmitted by UART 54 is loaded into the transmit FIFO 424by the microprocessor 18. As the transmit shift register 420 becomesempty it is reloaded from the FIFO 424. When the number of bytes in theFIFO 424 is equal to or less than a programmable threshold, the TransmitFIFO Threshold Reached Bit (5) is set in the Line Status Register (App.B). An interrupt is generated (if enabled) when the FIFO level FALLS TOthe programmed threshold level (the transition causes the interrupt, notthe level in the FIFO 404 being at or below the threshold). Thethreshold is programmed via bits 5 and 6 of the UART Control Register.Parity, if selected, is generated by element 428 connected to XMIT FIFO424 and XMIT Shift Register 420 as the data is moved from the FIFO 404to the shift register 420.

Frame generation by element 428 takes place only in the asynchronousmode of operation. The number of stop bits and character length areprogrammed into the UART transmitter. These parameters also hold for thereceiver. The number of stop bits is programmed in the Line ControlRegister bit 2. The character length is programmed by bits 0 and 1 ofthe Line Control Register.

Break Generation is also performed by element 428. Under control of themicroprocessor 18, the UART 54 will transmit an all ZEROs "Break"pattern until the Break bit is reset by the microprocessor 18. The Breakrequest bit is bit 6 in the Line Control Register. The UART 54 will waituntil any character currently being transmitted is complete beforetransmitting a break. (A minimum of ten contiguous ZERO bits will alwaysbe sent when a break is requested.) The UART transmitter will returnHIGH for at least one bit time following the transmission of a breakbefore a new character will be sent. This allows the start bit of thenew character to be detected. Break generation causes the transmit FIFO424 to be cleared.

The UART 54 controls until y 408 provides handshake signals for use incontrolling communications between the IDPC 10 and an external terminal.These signals are: RTS/, CTS/, DSR/, and DTR/. RTS/ and DTR/ are outputsto the terminal. They are controlled by the CPU bia bits 1 and 0 in theModem Control Register (App. B), respectively. CTS/ and DSR/ are inputsfrom the terminal. Their status can be read at Modem Status Registerbits 4 and 5 respectively. The CTS/ and DSR/ inputs generate a modemstatus interrupt if they have changed since the Modem Status Registerwas last read. This interrupt is enabled via Interrupt Enable Registerbit 3. The Delta CTS and Delta DRS bits in the Modem Status Register(0,1) reflect the fact that the status of CTS/ or DSR/ has changed sincethe Modem Status Register was last read. Reading the register clearsthese bits.

The UART 54 has four handshake lines in addition to the lines used bythe IDPC. These are Ring Indicate, (RI/), Receive Line Signal Detect(RLSD/), Output 1, and Output 2. RI/ and RLSD/ are inputs to the UART54. Their status is reported by bits 6 and 7 of the Modem StatusRegister, respectively. An interrupt is generated by the detection ofthe trailing edge of RI/ (RI/ going inactive-high). This is reported viabit 2 in the Modem Status Register. An interrupt is generated if RLSD/changes since the Modem Status Register was last read. This Delta RLSD/indication is reported via bit 3 in the Modem Status Register. TheOutput 1 and 2 signals are general purose outputs. They are controlledby setting and clearing bits 2 and 3 in the Modem Control Register,respectively.

With reference again to FIG. 2, a dual-port timing controller (DPTC) 56illustrated connected to the MPI 50 via a control line 57 and to theIDPC internal busses 58, 60 and 62. Also shown in FIG. 2 connected toDPTC 56 are signal lines conducting signals to and from externalterminal pins of the IDPC 10. These include bus arbitration controlterminals: a local processor bus request (LREQ/) input terminal, a localdata transmit/Receive (LDT-R/) input terminal, a local ready (LRDY)output terminal, a host processor bus request (HREQ) input terminal, ahost data transmit/receive (HDT-R) input terminal, a host ready (HRDY)output terminal, a host interrupt in (HINTIN) input terminal, a hostinterrupt acknowledge (HINTACK) input terminal, a host interrupt out(HINTOUT) output terminal, a local interrupt out (LINTOUT) outputterminal. Also a set of four host port (HPORT) control output lines, aset of three RAM control output lines and four local port (LPORT)control output lines. The HPORT lines include a Host Data Latch Enable(HDLE) output line, a host data bus enable (HDBE/) output line, a hostaddress bus enable (HABE/) output line, and a host data laach outputenable (HDLOE/) output line. The RAM control lines include a RAM ChipSelect (RAMCS/) output line, a RAM write enable (RAMWE/) output line anda RAM Output enable (RAMOE/) output line. The LPORT output lines includea local address bus enable (LABE/) output line, a local data abus enable(LDBE/) output line, a local data latch enable (LDLE) output line and alocal data latch output enable (LDLOE/) output line.

The signals on the various lines connected to the DPTC56 are nowdescribed more completely.

The input pin is used as the direction control for the DPTC 56 portionof the IDPC when the local processor 18 is accessing a shared RAM to bedescribe hereinafter in connection with FIG. 23. When this pin goes highit indictes that a write cycle to the shared RAM is in progress. As aresult of LDT-R/ going high, RAMWE/ and LDBE/ are driven active (low).When LDT-R goes low it indicates that a read cycle from the RAM is inprogress. At this time RAMOE/, LDLE/ and LDLOE/ are driven active low.LDT-R is returned to the inactive state on the next falling edge of CLK.

HDT-R/ functions identically to LDT-R except that it is the directioncontrol supplied when a remote host is accessing the shared RAM.

The LREQ/ is an active low input to the IDPC 10 from the local processor18 requesting access to the shared RAM. LREQ/ is sampled on the negativeedge of every IDPC clock cycle. LREQ/ is normally active for two IDPCclock cycles. When sampled active, LREQ/ drives RAMCS/ and LABE/ active(low). LREQ/ is to be synchronous to the clock.

The HREQ signal functions identically to LREQ/ except that it comes froma remote host processor requesting access to the shared RAM and it isactive high. HREQ is an asynchronous input with respect to the IDPCclock.

LRDY is an active high output from the IDPC 10 used by the localprocessor 18 to complete shared RAM memory cycle. LRDY is normally high.It is driven low when request for the shared RAM is received from thelocal processor 18(LREQ/) and the host processor is currently accessingthe shared RAM.

The HRDY signal performs the same functions as the LRDY except that itis used by the host processor.

When activated LINTOUT signal intended to generate and interrupt to thelocal processor 18. LINTOUT goes active (high) as a result of bit 1 in aSemaphore Register being set to a 1. LINOUT is returned to the inactivestate when bit 1 of the Semaphore Register is cleared to 0.

The HINTOUT pin functions similar to the LINTOUT pin but it is intendedto interrupt the host processor. HINTOUT is activated by the localprocessor 18 writing a done into bit 0 of the Semaphore Register.HINTOUT is intended to be connected to an interrupt input on the hostprocessor. HINTOUT is deactivated by the host pulsing the HINTACK pinwhich clears bit 0 of the Semaphore Register and deactivates HINTOUT.

The HINTIN is an active high input used by the host processor togenerate an interrupt to the local processor 18 (LINTOUT). When HINTINgoes active it causes bit 1 of the Semaphore Register to be set to onewhich in turn generates LINTOUT out. This mechanism is necessary becausethe host cannot read/write the Semaphore Register.

The HINTACK active high input to the IDPC 10 clears bit 0 of theSemaphore Register to a zero. HINTACK is output by the host processor inresponse to the receipt of a HINTOUT interrupt from the IDPC.

LABE/ is driven active low by the IDPC 10 as a result of receiving anLREQ/ from the local processor 18 and is used to enable the addresslines from the local processor 18. LABE/ remains active until the end ofthe memory cycle.

HABE/ functions identically to LABE/ except that it enables addresslines from the host address latches to the memory bus and is activatedby HREQ.

LDBE/ is an active low output used to enable the data lines from thelocal processor 18 to the shared RAM data bus. LDBE/ is driven active asa result of LDT-R/ being driven high (write cycle). It remains highuntil the end of the memory cycle.

HDBE/ functions identically to LDBE/ except that it enables data fromthe host bus onto the shared RAM bus and it is activated by HDT-R/ goinghigh.

The LDLE active high output is used to latch data from the shared RAM tothe local processor 18. LDLE is driven low (the latch is madetransparent) as a result of LDT-R/ going low (read cycle). It returnsactive (high) at the end of the memory cycle.

HDLE functions identically to LDLE except that it latches data from theshared RAM 22A to the host processor data bus and is activated by HDT-R/going low (read cycle).

LDLOE/ is an active low output from the IDPC 10 used by the localprocessor 18 to enable the output of the data bus latches back to thelocal processor 18. LDLOE/ is driven active (low) when LDT-R/ is drivenlow (read cycle). It is cleared (high) when LREQ/ goes inactive.

HDLOE/ functions identically to IDLOE/ except that it is used to enablethe output of data onto the host data bus and is activated (low) byHDT-R/ going low (read cycle). It is cleared (high) when HREQ goesinactive.

RAMCS/ is an active low output from the IDPC 10 used by the shared RAM22A as its chip select enable. It is driven active (low) when eitherLREQ/ or HREQ are sampled active. RAMCS/ remains active until the end ofthe memory cycle.

RANWE/ is an active low output from the IDPC 10 used by the shared RAMas a write strobe. It is activated when either LDT-R/ or HDT-R/ goeshigh (write cycle).

It is deactivated at the end of the memory cycle.

RAMOE/ is an active low output signal used by the shared RAM 22A toenable its output drivers. It is driven active when either LDT-R/ orHDT-R/ is driven low (read cycle). It is cleared (high) at the end ofthe memory cycle.

With reference now to FIG. 23, the IDPC 10 can be used in host-basedsystems in which the "local" microprocessor 18 and an external "host"communicate with one another via a shared memory 22a (dual port RAM).This memory is an external shared RAM (SRAM), that can be accessed byeither the microprocessor 18 or the host. The Dual Port TimingController (DPTC) 56 of IDPC 10 provides the control function necessaryto allow an ordinary SRAM 220 to function as a dual port device. Thesefunctions include: memory cycle timing generation, control of buffersand latches required to isolate the host's system bus 500 from themicroprocessor 18 local bus and generation of the Ready control signalsback to the host and the local processor.

In addition to arbitrating accesses to the shared RAM 22a, the DPTC 10provides a semaphore mechanism (bidirectional Interprocessor Interrupts)that is used to coordinate the passing of high level messages to andfrom the local microprocessor 18 and the host. SRAM 22a is connected tothe data and address busses 26 and 28 and receives the RAMCS/ (at CS/terminal), the RAMOE/ (at an OE/ terminal) and the REMWE/ (at aWE/terminal) and WE/ generated by the DPTC 56 of the IDPC 10.

Various bus isolation devices are shown in FIG. 22. A latch 502 isplaced on the data bus 26 between the SRAM 22a and the IDPC 10. Latch502 receives the LDLE at an OE/ input and LDLE/ at an E input from theDPTC 56 of the IDPC 10. A latch 504 is placed on the data bus 26 betweenthe IDPC 10 and the SRAM 22a receives the LDBE/ at an OE/ input from theDPTC 56. A latch 506 is placed on the address bus 28 between the IDPC 10and the SRAM 22a receives the LABE/ at an OE/ input. A latch 508 isplaced on the address bus 28 following the latch 506 between the hostsystem bus 500 and the SRAM 22a receives the HABE/ signal at an OE/terminal generated by DPTC 56. A latch 510 is placed on the data bus 26following the latched 502 and 504 between the host system bus 500 andreceives the HDBE/ signal at an OE/ terminal generated by the DPTC 56. Alatch 512 is placed on the data bus 26 following the latches 502 and 504between the SRAM 22a and the host system bus 500 and receives the HDLOE/signal at an OE/ terminal and a HDLE signal at an E terminal generatedby the DPTC 56.

Finally, the IDPC 10 communicates the HINTOUT, HINTIN, HINTACK, HRDY,HDT-R/ and HREQ signals to and from the host system bus 500 and theLINTOUT, LREQ/, LRDY, LDT-Rl conducted to the MCSO, SRDY and SI/terminals of local processor 18.

With reference now to FIG. 24, a functional block diagram of the DPTC 56of IDPC 10 shows reception and generation of the various control signalsdescribed hereinbefore in connection with FIG. 2. The operation of DPTC56 will be described in connection with the seven major functionalblocks shown in FIG. 24.

A synchronization (SYNC) block 520 receives the master clock (CLK)signal from the local processor 18, as well as the HREQ signal from thehost on bus 500. The synchronization block 520 generates a host requestsignal synchronized with the local clock which is conducted to aconflicting request cycle arbitration block 522 which also receives theLREQ signal from local processor 18 and the CLK signal. Cyclearbitration block 522 generates a local cycle (LCYCLE) signal and a hostcycle (HCYCLE) signal therefrom for use by the other functional blockswithin DPTC 56. Cycle arbitration block 522 also generates a GO signalreceived by a RAM cycle time block 524. RAM cycle timer block 524 alsoreceives the CLK signal and the LDT-R/ and HDT-R/ signals, and generatestherefrom the RAMOE/, the RAMCS/ and the RAMWE/ signals, as well as aSTOP signal.

A local port cycle controller 526 receives the STOP signal generated byRAM cycle timer 524, the CLK signal, the LCYCLE signal and the LDT-Rsignal and generates therefrom the LDLE, LDLOE/, LDBE/ and LABE/signals.

A host port cycle controller 528 receives the STOP signal, the CLKsignal, the HCYCLE signal and the HDT-R/ signal and generates therefromthe HDLE, HDLOE/, HDBE/ and HABE/ signals.

A local port ready (LRDY) controller 530 receives the LREQ signal, theCLK signal, the HCYCLE signal and the LCYCLE signal and generatestherefrom the LRDY signal.

A host port ready (HRDY) controller 532 receives the HREQ signal, theCLK signal, the HCYCLE signal and the LCYCLE signal and generatestherefrom the HRDY signal.

Design and construction of the various blocks shown in FIG. 24 will beunderstood by those skilled in the art when taken with their descriptionabove as well as further description of their functioning given now inconnection with the DPTC timing diagram of FIG. 25.

The DPTC 56 generates the cycle timing for all accesses to the sharedRAM 22a. The length of each cycle is fixed, and a independent of thecycle times of either the local processor 18 or the host. Memory cyclesare generated in response to a request from either the local processor18 or the host. In the case of conflicting requests the DPTC 56arbirates the conflict, granting the first cycle to one requester whileholding off the other (via the appropriate Ready line LRDY or HRDY). TheDPTC 56 will always arbitrate in favor of the local processor 18(referred to as the L-port). With reference now to the timing diagram ofFIG. 25, if the L-port has a request pending (via the LREQ/ input) atthe time when the cycle arbitration block 522 is ready to start the nextmemory cycle, at the time labelled by reference numeral 540 in FIG. 25,the L-port will be granted the cycle regardless of a request from thehost (H-port). If a request from the host (HREQ input pin) is present,or becomes present during the cycle (L-cycle) time 542, the next cyclewill be granted to the host (H-cycle) time 544. This takes placeimplicitly since the DPTC 56 will start the next cycle before the localprocessor 18 can request another cycle time 546. If an L-cycle requestis received in the middle of an H-cycle the local processor 18 is heldoff (via the LRDY line) until the H-cycle has completed (time 548through time 550).

L-cycle requests are synchronous with the IDPC clock. This is not aproblem since the IDPC clock is the same timing is generated from theIDPC clock. H-cycle requests are assumed to be asynchronous of the IDPCclock, and are synchronized internally to the DPTC within the SYNCHblock 520.

As shown in FIG. 25, the SRAM 22a memory cycle is two IDPC clock (CLK)times in length, with at least one CLK clock time dead space in betweenany two SRAM cycles.

While the SRAM 2a is idle the cycle arbitration block 522 of DPTC 56samples the LREQ/ and synchronized HREQ signals on the falling edge ofevery IDPC clock cycle. If a request is present a cycle is started and aGO signal is generated to RAM cycle timer 524. The starting of a cyclecauses the following actions to take place: RAMCS/ is driven active(low) by RAM cycle timer 524 (time 552) and either LABE/ or HABE isdriven active (low) (times 554 or 556, respectively) depending onwhether LREQ/ or HREQ/ was sampled.

RAMCS/ provides the chip select control output to the SRAM 22a. BothRAMCS/ and either LABE/ or HABE/ signals remain active until the end ofthe memory cycle times 558 or 560 respectively.

On the next falling edge of the IDPC clock (CLK) the active port'sdirection control input line (LDT-R/ or HDT-R) is sampled (times 562 or564, respectively) by RAM cycle timer 524. This signal determineswhether the cycle is a read or write cycle. If the direction control issampled high (write) the following actions are taken: RAMWE/ is drivenactive (low) by RAM cycle timer 524 (time 566) and either LDBE/ or HDBE/is driven active (low) (times 568 or 570, respectively) depending onwhether LDT-R/ or HDT-R/ was sampled.

RAMWE/ is the SRAM 22a write strobe. It is returned to its inactive(high) state at the end of the cycle (time 572). LDBE/ and HDBE/ are thedata buffer enable controls that place the data to be written into theSRAM 22a on the memory bus. They are also returned to their inactive(high) state at the end of the cycle (time 574 or 576, respectively).

If the direction control line LDT-R/ or HDT-R/ is sampled low (read),the following happens: RAMOE/ is driven active (low) by RAM cycle timer524 (time 578); LDLE or HDLE is driven active (low) (times 580, time forHDLE omitted for clarity); and LDLOE/ or HDLOE/ is driven active (LOW)(time 582, time for HDLOE/ omitted for clarity).

RAMOE/ enables the SRAM 22a output drivers. LDLE and HDLE place theappropriate data bus latch 502 or 512, respectively, in its transparentstate. LDLOE/ and HDLOE/ enable the data abus latch 502 or 512,respectively, outputs back to the local or host system bus. RAMOE/,LDLE, and HDLE are cleared at the end of the cycle (times 584, 586,respectively, time for HDLE omitted for clarity). LDLOE/ (time 588) andHDLOE/ (time omitted for clarity) are cleared when the cycle request(LREQ/ or HREQ) is removed (time 590).

The memory cycle ends on the next falling edge of the IDPC (CLK) clock(time 592).

The LREQ/ and HREQ inputs are sampled by cycle arbitration block 522 oneach successive falling edge of the IDPC clock (CLK) to determine if anew cycle is to be started.

The LCYCLE adn HCYCLE control blocks 526 and 528 generate the variousLDLE, LDLOE/, LDBE/, LABE, and HDLE, HDLOE/, HDBE/, HABE timing signalsdescribed in connection with FIG. 24.

In the event that the L-port requests a cycle while an H-cycle is inprogress, or the H-port requests a cycle while either an L-cycle is inprogress or an L-port request is present, a conflict occurs. The cyclearbitration block 522 of DPTC 56 will always arbitrate in favor of theL-port.

If LREQ/ becomes active while an H-cycle is in progress LRDY is driveninactive (low) by LRDY control 530. This takes place immediately. LRDYis returned active at the start of the next memory cycle (which will bean L-cycle).

The case in which HREQ becomes active while an L-cycle is in progress inhandled exactly the same as above, except the HRDY is used as thecontrol signal instead of LRDY.

The case where HREQ is active prior to the start of a cycle and LREQ/also becomes active, causes HRDY to be driven inactive (low) as soon asLREQ/ becomes active. (If LREQ/ is already active--before the L-cyclestarts--HRDY is driven inactive as soon as HREQ becomes active.) HRDY isreturned active when the H-cycle is started.

With reference now to FIG. 26, all communication between the localprocessor 18 and the host processor 595 takes place through "mailboxes"located in shared RAM 22a. A mechanism is required to inform therecipient that there is a message in his mailbox. Interrupts are usedfor this task.

Message passing takes two forms: local processor 18 sending to the host595, and host sending to the local processor 18. When the localprocessor wishes to send a message to the host 595, it first placed themessage in the host's mailbox and then generates an interrupt request tothe host. The host reads the message and clears the interrupt request.Conversely, when the host wishes to send a message to the localprocessor 18 it places the message in the local processor's mailbox andgenerates an interrupt request to the local processor 18. The localprocessor 18 reads the message and clears the interrupt request. TheDPTC 56 provides a two-bit semaphore register 596 to facilitate thegeneration and clearing of these interrupt requests. The low-order bitposition of samaphore register 596 (bit 0) contains an interrupt-to-hostprocessor (ITLP) flag and the high-order bit postion (bit 1) contains aninterrupt-to-local processor (ITHP) flag.

The ITLP bit protion of semaphore register 596 is connected to the localprocessor 18 (via a external pin of IDPC 10) by a signal line 597carrying a LOCAL INT OUT signal. The host processor 597 is connected tothe ITLP bit position (via an external pin of IDPC (0) by a signal line598 carrying a HOST INT IN signal. The local processor 18 can write aLOCAL IN CLR signal to bit 1 of register 596 and a HOST IN REQUESTsignal to bit 0 of register 596 via the MPI 50.

The host 595 is connected to the ITHP bit position of semaphore register596 (via an external pin of IDPC 10) by a signal line 599a carrying aHOST IN ACK signal and (via an external pin of IDPC 10) by a signal line599b carrying a HOST IN OUT signal.

The semaphore Register 596 can be read and written by the local procesor(but not by the host). Local processor-to-host interrup--The localprocessor 18 generates an interrupt to the host 595 by writing a ONE tobit 0 in the Semaphore Register 596. The setting of this bit activatesthe host interrupt output (HINTOUT pin). The host 595 clears the bit,and therefore the HINTOUT pin, by pulsing the host interrupt acknowledgeinput (HINTACK pin).

Host-to-local-processor interrupt--The host 596 generates an interrup tothe local procesor 18 by pulsing the host interrupt input (HINTIN pin).This sets bit 1 in the Semaphore Register 596 and activates the localinterrupt output (LINTOUT pin). The local processor clears the interruptrequest (generated by the LINTOUT line) by clearing bit 1 in theSemaphore Register.

The IDPC 10 of the present invention contains a number of registers intowhich user-definable data can be written and/or from which data can beread. The DLC 52 transmitter 102 has a set of status and controlregisters (112, FIG. 4), the DLC receiver 108 has a set of status andcontrol registers (212, FIG. 10), the UART 54 has a set of status andcontrol registers (408, FIG. 21) and a set of special character bit mapregisters within element 412 and the DPTC 56 has a single semaphoreregister. These registers are accociated with various aspects of each ofthese three major functional blocks of the IDPC 10, as will now bedescribed.

The user accesses the various status and control registers and thesemaphore register 596 via microprocessor 18 by supplying an address tothe MPI 50 according to the following memory map:

    ______________________________________                                        Address             Use                                                       ______________________________________                                        00-31               DLC 52                                                    32-62               UART 54                                                   63                  DPTC 56                                                   ______________________________________                                    

Status and control registers listed in Table I below are containedwithin the DLC 52 in blocks 112 and 212 and are used to establish therequired modes and configurations of the DLC transmitter 102 andreceiver 108 as well as monitoring and reporting the necessary status ofthe DLC 52 to the user. The status and control registers used for theDLC FIFOs 100 and 106 and Serial Bus Port (SBP) 104 are also listed inTable I. These rgisters occupy the first 29 locations in a 32 byte DLCaddress space. This space starts at location 00 in the Internal MemoryMap.

Prior to discussing the details of the individual registers listed belowin Table I, the following section describes two DLC registers and a bitfield in a third register which differ from the other user-accessibleDLC registers.

The Receive Frame Status Register, Receive Byte Count Register, and theReceive Link Address Bit field (bits 0-2) of the Interrupt SourceRegister support the reception of multiple contiguous frames(back-to-back frames). These two registers and the bit field of thethird register must maintain a "history" of the status (good or badframe) and byte count of up to two previously received frames while athird frame is actually being received by the DLC 52. These registersand the bit field are four stage registers to support multiple levels ofstatus from frame received by DLC 52.

Each of the Receive Frame Status Register, Receive Byte Count Registerand the Receive Link Address Bit Field are comprised of the followingfour stages:1) Current, 2) Holding, 3) Master, and 4) Slave. A typicalfour-stage "delayed-status" structure used for the Receive Frame StatusRegister is shown in FIG. 27. The FIG. 27 illustrates a typicalinterconnection as will be described hereinafter.

With reference to FIG. 27, the flag detector 214 of DLC receiver 108 isconnected to an AND gate 600 as is the eight-bit minimum (receive)packet size register 264 (together representing "stage 1"). (Althoughonly and AND gate 600 is shown in FIG. 27, eight such gates, operationin parallel and each connected to the flag detector, are employed.) Thesignals generated by AND gates 600 are conducted to an eight-bit latch602, receiving at a reset (R) input the IN-FRAME signal. The outputs oflatch 602 are conducted to a set of (eight) AND gates 604, each alsoreceiving an OUT-OF-FRAME signal and an EMPTY signal generated by aneight-bit ("stage 2") register 606, Register 606 receives the eightsignals conducted in parallel from AND gates 604.

Register 606 is, in turn, connected at its outputs to a set of (eight)AND gates 608, each also receiving an EMPTY signal generated by aneight-bit ("stage 3") register 610. Register 610 is, in turn, connectedat its outputs to a set of (eight) AND gates 612, each also receiving anEMPTY signal generated by an eight-bit ("stage 4") register 614.Register 614, is, in turn, connected at its outpus to set of (eight) ANDgates 616 each also receiving a signal from bit 5 of a Receive FrameStatus Interrupt Enable Register (typical of other bit positions withinthe register). The outputs of AND gates 616 are conducted in parallel toan OR gate 618. The single output generated by OR gate 618 is conductedto bit position five of an Interrupt Souce Register 620, which is, inturn, connected at is output to an AND gate 622. AND gate 622 alsoreceives a signal from bit 5 of an Interrupt Source Interrupt EnableRegister. The signal generated by AND gate 622 generates a DLC interruptsignal.

Stage 1 (602) contains the current status of the frame presently beingreceived by the DLC 52. This stage is updated with the current DLCreceiver status as changes occur in real-time. The contents of Stage 1is transferred to Stage 2 (606) via gates 604 when Stage 2 is empty andan event occurs that caused a byte to be tagged as the End-of-Frame(EOF) byte as it enters the DLC Receive FIFO 106 as describedhereinabove in connection with FIG. 18. If Stage 2 is not empty (i.e.,contents not yet transferred to Stage 3), then Stage 1 will retain itsstatus. When Stage 1 is transferred to Stage 2, Stage 1 is free to beginmonitoring the next frame that arrives. Stage 2 then holds the datauntil Stage 3 is empty. Data is transferred from Stage 2 to Stage 3 assoon as Stage 3 becomes empty. This, in turn, allows transfer of datafrom stage 1 to Stage 2 at the EOF condition.

The contents of Stage 3 register 610 is transferred to Stage 4 register614 (which is the register accessible by the user) when Stage 4 is emptyand the EOF-tagged byte is read (by DMA or the microprocessor 18) fromthe Receive FIFO Data Register 298. If Stage 4 is not empty, then Stage3 will retain its status. Stage 4 is cleared ("emptied") when read bymicroprocessor 18 or when DLC 52 Reset occurs.

Received frame status can "back up". If microprocessor 18 has not readStage 4 when an event occurs that would cause Stage 3 to be transferredto Stage 4, Stage 3 is not transferred to Stage 4. There is no problemif Stage 4 is emptied before an event occurs which would cause Stage 1to be transferred to Stage 2. In this case, Stage 3 is transferred toStage 4 as soon as Stage 4 is emptied. However, with both Stages 2, 3,and 4 non-empty when an event occurs that would result in Stage 1 beingtransferred to Stage 2, existing Stages 1, 2, 3, and 4 are notdisturbed. The DLC Receiver 108 begins ignoring all received link bytes,freezing Stage 1. Any frames which are transmitted to the DLC receiverare therefore lost until Stage 4 is read by microprocessor 18. As soonas Stage 4 is emptied by a microprocessor read, Stage 3 is transferredto Stage 4, Stage 2 is transferred to Stage 3 and Stage 1 is transferredto Stage 2. At this point, the DLC Receiver 108 logic enters ReceiverState 0 (Hunt for Flag) and frame reception can begin again.

The valid and invalid packet received bits (3,5) of the Interrupt SourceRegister are also reported in a delayed fashion. These bits can be builtfrom the other delayed status conditions and do not themselves requirefour-stage mechanism.

If Receive Frame Status Register has not been read since the InterruptSource Register was last read, and the least significant bit of theReceive Byte Count Register is read, the Receive Status Register will becleared. In this manner, the four-stage delayed status mechanism remainssynchronized if a valid data packet is received and the Receive StatusRegister is not read.

The DLC 52 contains a number of registers within the transmitter statusand control registers block 112 (FIG. 4) and the receiver status andcontrol registers block 212 (FIG. 10). These registers are listed inTable I.

                  TABLE I                                                         ______________________________________                                        DLC 52 Status and Control Registers                                           IDPC Hex                         Length                                       Address Register                 (Bytes)                                      ______________________________________                                        00      Command/Control Register 1                                            01      DLC Address Control Register                                                                           1                                            02      Link Address Recognition Register 0                                                                    2                                            04      Link Address Recognition Register 1                                                                    2                                            06      Link Address Recognition Register 2                                                                    2                                            08      Link Address Recognition Register 3                                                                    2                                            0A      Serial Bus Port (SBP) Control Register                                                                 1                                            0B      Minimum Receive Packet Size Register                                                                   1                                            0C      Maximum Receive Packet Size Register                                                                   2                                            0E      Interrupt Source Interrupt Enable Register                                                             1                                            0F      Receive Frame Interrupt Enable Register                                                                1                                            10      Receive Link lnterrupt Enable Register                                                                 1                                            11      FIFO Status Interrupt Enable Register                                                                  1                                            12      Transmit Byte Count Register                                                                           2                                            14      FIFO Threshold Register  1                                            15      Interrupt Source Register                                                                              1                                            16      Receive Byte Count Register                                                                            2                                            18      Receive Frame Status Register                                                                          1                                            19      Receive Link Status Register                                                                           1                                            1A      FIFO Status Register     1                                            1B      Receive FIFO Data Register                                                                             1                                            1C      Transmit FIFO Data Register                                                                            1                                            ______________________________________                                    

Appendix A contains a description of the DLC status and controlregisters listed in Table I.

The DLC registers fall into five categories, command/control, status,FIFO data, interrupt identification, and interrupt enable registers.

DLC command/control regiters--The command/control registers are used toconfigure the DLC, and to request specific actions. These include:

DLC Command/Control Register;

Link Address Control Registers (4);

SBP Control Register;

Minimum Receive Packet Size Register;

Maximum Receive Packet Size Register;

FIFO Threshold Register; and

Transmit Byte Count Register.

The DLC Command/Control Register controls the operation of the entireDLC. The other registers each control the actions of a specific portionof the DLC. The bit assignments are not critical in these registers.

DLC status registers--The DLC status registers report the state of theDLC to the user. These include:

FIFO Status Register;

Receive Byte Count Register;

Receive Frame Status Register;

Receive Link Status Register; and

Portions of the Interrupt Source Register.

The status registers are organized to group status information accordingto type. This is important since this is the way that the softwareinterrupt handling routines will tend to be organized. For example,status information that pertains to the real time condition of thecommunications link is reported separately from information specific toa particular received frame of data. In general, the software isinterested in the status of packets, not portions of packets--this isthe key, the DLC reports status to the user only after the packet hasbeen moved from the IDPC into external RAM. Traditionally, the user wasinvolved in either the status of data on a character by character orseveral character at a time basis, not on a packet at a time basis. Thisstatus structure is unique to the IDPC.

The bits inside the various status registers are organized with the mostprobable condition in the LSB position and the least probable conditionin the MSB position. This reduces the software overhead in identifying agiven condition. Further, the most likely status conditions (and theidentification of the adress associated with the packet in question) arereported in the Interrupt Source Register. This is the first registerthat a user will read after receiving an interrupt. Under all normalconditions, the user need access no other register to identify thestatus of the DLC.

FIFO data registers--These are the registers that the user reads orwrites to move data to and from the DLC (FIFOs).

Interrupt identification register--The Interrupt Source Registercontains three bit fields, the packet address identification field, thevalid packet field, and the interrupt source field. The first two fieldswere discussed above. The interrupt source field has one bit for each ofthe status registers. When this bit is set, the associated registercontains the interrupting condition. In this manner, the user canefficiently locate the cause of the interrupt.

By way of example: To identify that a valid packet has been received,the IDPC requires only a read to the Interrupt Source Register, and ashift right and test instruction--a total of two instructions. If a lessoptimum organization were used, as many as 17 instructions could havebeen required.

Interrupt enable registers--These registers are bit-for-bit matches ofthe status registers. This provides a user friendly organization forinterrupt enabling. Extra registers are required by this scheme, but theresult is clearer for the user.

Table II lists the 12 user-accessible status and control registers inthe UART 54 plus a 128 bit bit-map, addressed as 16, 8 bit registerscorresponding to the random access memory 413 of the Parity, SpecialCharacter, Frame, Break Checker 412. The UART registers are mapped intoa 31 byte space. The two baud rate divisor registers overlap the FIFOData Registers and the Interrupt Enable Register Addresses. Access isgained by setting the Divisor Latch Access Bit (DLAB) in the LineControl Register. The base address of the 31 byte block is indicated inthe memory map presented earlier.

                                      TABLE II                                    __________________________________________________________________________    UART 54 Status and Control Registers                                          Address Register Name       Type                                              __________________________________________________________________________    0 DLAB=0*                                                                             RECEIVE FIFO DATA REGISTER                                                                        Read                                              0 DLAB=0*                                                                             TRANSMIT FIFO DATA REGISTER                                                                       WRITE                                             0 DLAB=1*                                                                             BAUD RATE DIVISOR LSB                                                                             READ/WRITE                                        1 DLAB=0*                                                                             INTERRUPT ENABLE    READ/WRITE                                        1 DLAB=1*                                                                             BAUD RATE DIVISOR MSB                                                                             READ/WRITE                                        2       INTERRUPT IDENTIFICATION                                                                          READ                                              3       LINE CONTROL        READ/WRITE                                        4       MODEM CONTROL       READ/WRITE                                        5       LINE STATUS         READ                                              6       MODEM STATUS        READ                                              7       UART STATUS         READ                                              8       UART CONTOL         READ/WRITE                                        9-24    EIGHT BIT SPECIAL                                                             CHARACTER BIT MAP REGISTERS                                                                       READ/WRITE                                        __________________________________________________________________________     *DLAB bit is defined above.                                              

Appendix B contains a description of the UART status and controlregisters listed in Table II.

UART registers--The UART registers are identical to those defined in the8250, with the exception of the bits required to control and reportstatus on the features added to the 8250 (special character recognition,FIFOs, synchronous operation, etc.). The trick here is to be exactlycompatible with the 8250, while providing the new capabilities cleanly.The UART Status, and UART Control registers are new to the basic 8250.Additional bits have been added to unused positions in existing 8250registers (identified in the UART register section of the FPS). Thepositions of these added bits is important to direct the interruptsource identification process in a logical manner.

The DPTC 56 contains one user-accessible register. This register is usedby the local processor 18 to control the generation and clearing ofsemaphore interrupts between the local processor 18 and the hostprocessor 595. This DPTC register is described hereinabove in connectionwith FIG. 26. The DPTC register (Semaphore Register 596) is mapped intoaddress 63 decimal.

APPENDIX A DLC 52 Status/Control Registers (112,212)

COMMAND/CONTROL REGISTER is an eight-bit register. All bits in thisregister are set and cleared by software except when initialized todefault values as a result of DLC Reset as described hereinbelow or IDPCReset Pin as described hereinbelow. This register can be written to andread from by the microprocessor local processor 18. ##STR1##

Bit 0: Send Abort (Default=0)

When this bit is set to ONE, the following actions result immediatelyand persist until the bit is cleared to ZERO:

a) DLC transmitter Flag/Abort Insertion Unit 134 (FIG. 7B) transmitsAbort characters (bit pattern 01111111 (LSB on right)).

If the user via microprocessor 18 sets and clears this bit on twosuccessive writes the DLC will transmit one "Abort" character.

b) Clear the DLC 52 Transmit FIFO 100 (FIG. 4).

c) Clear the DLC Transmit Byte Counter 154 (FIG. 5).

d) Clear the DLC Transmit Byte Count Register 152 (FIG. 5).

Bit 1: Transmitter Enable (Default=0)

When set to ONE, this bit allows data from the DLC 52 to be shifted outto SBOUT (FIG. 8) under control of the SCLK or SFS/XMITCLK. When thisbit is cleared to ZERO, the SBOUT pin is placed in a three-statecondition. When this bit is cleared to ZERO and the DLC transmitter 102is "In-frame" (state 1 (302) of FIG. 19), i.e., transmitting data, theDLC 52 waits until the current frame is complete (i.e., DLC transmitterout of frame) before disabling the SBOUT pin.

Bit 2: Receiver Enable (Default=0)

This bit, when set to ONE, allows data from the SBIN pin to be clockedinto the Serial Bus Port (SBP) portion 104 of the DLC 52. When clearedto ZERO, this bit blocks the receipt of any data into the SBP portion ofthe DLC. If this bit is cleared while the DLC receiver 108 is In-frame,the DLC 52 will wait for the frame currently being received to terminatein an orderly fashion (i.e., Closing Flag received or terminating erroroccurs) before disconnecting.

Bit 3: Flag Idle-Mark Idle/(Default=0)

This bit, when set to ONE, causes the DLC transmitter 102 tocontinuously transmit the Flag Idle pattern when not In-frame. Whencleared to ZERO, this bit causes the DLC transmitter 102 to continuouslytransmit the Mark Idle pattern when not In-frame.

Bit 4: CRC Check Enable (Default=1)

When set to ONE, this bit allows the output of the CRC Check resultgenerated by CRC checker 222 to be transferred to the CRC Error bit (bit2) in the Receive Frame Status Register. When this bit is cleared toZERO, the CRC Error bit in the Receive Frame Status Register is neverset.

Bit 5: CRC Generate Enable (Default=1)

When set to ONE, this bit causes the transmit CRC generated by CRCgenerator 120 (which is always being calculated) to be transmittedfollowing transmission of the byte tagged as End-of-Frame (EOF) in theDLC Transmit FIFO 100 (FIG. 4). When this bit is cleared to ZERO, aClosing Flag is generated by FLAG, ABORT INSERTION generator 134 andtransmitted immediately following the byte tagged as EOF, and the FCS isnot sent.

Bit 6: DLC Reset (Default=0)

This bit, when set to ONE, resets the DLC FIFOs 100 and 106 and DLC 52and SBP 104 logic. All latches, status and control bits in the DLCstatus and control registers 112, 212 are forced to their defaultvalues.

Bit 7: FCS Pass-through Enable (Default=0)

When set to ONE, this bit allows the FCS bytes to be loaded into theFIFO 106 as data (receive side). When cleared to ZERO, the FCS isdiscarded.

DLC ADDRESS CONTROL REGISTER is an eight-bit register. All bits in thisregister are set and cleared by software except when initialized todefault values as a result of DLC Reset described in connection with DLCCommand/Control Register or IDPC Reset Pin. This register can be writtenand read by the microprocessor 18. When all Link Address Enable bits(bits 0-3) and the Broadcast Enable bit (bit 4) are cleared to ZERO, theDLC will perform no address detection and pass all received frame bytes(assuming more than two frame bytes are received) to the DLC ReceiveFIFO 106 (FIG. 10). In this case, bits 5, 6 and 7 of this register areignored.

If one or more of bits 0-4 are set to one, then a successful linkaddress compare described in connection with Address Detection Unit 226(FIG. 16) must occur before any frame bytes can be transferred to theDLC Receive FIFO 106. ##STR2##

Bit 0: Link Address 0 Enable (Default=0)

Bit 1: Link Address 1 Enable (Default=0)

Bit 2: Link Address 2 Enable (Default=0)

Bit 3: Link Address 3 Enable (Default=0)

When set to ONE, bits 0 through 3 enable comparison of a received frameaddress with the contents of DLC Link Address Recognition Registers 0through 3, respectively, of blocks 278, 280, 282 and 284 (FIG. 17). Thecontents of a given Link Address Recognition Register should have beenwritten by software before software sets the corresponding Link AddressEnable bit in this register. The comparison of a received frame addresswith the contents of all enabled Address Recognition Registers isconditioned by bits 5 and 6 of this register as described hereinbelow.

Bit 4: Broadcast Address Enable (Default=1)

When set to ONE, this bit enables comparison by comparators of blocks278, 280, 282 and 284 of a received frame address with an all 1'saddress. The comparison is conditioned by bits 5 and 6 of this registeras described hereinbelow. When cleared to ZERO along with bits 0-3 ofthis register, the DLC performs no address detection. If cleared to ZEROand one or more bits of 0-3 are set to ONE, then the all 1's patternaddress is ignored.

Bit 5: Address size 1-2 (Default=0)

At least one of the bits 0-4 in this register must be set to ONE forthis bit to have any effect on DLC 52 operation. Given that, if this bitis cleared to ZERO, two frame address bytes must be compared for addressrecognition to occur in block 226. If this bit is set to ONE, only thefirst frame address byte must be compared for address recognition tooccur by address detection unit 226. Bit 7 specifies whether the firstor second byte is the one that is compared.

Bit 6: C/R Address Enable (Default=0)

At least one of the bits 0-4 in this register must be set to ONE forthis bit to have any effect on DLC 52 operation. Given that, if this bitis cleared to ZERO, then bit 1 of the first address byte of eachreceived frame will be ignored for the purposes of address recognitionby clock 226. If this bit is set to ONE, then bit 1 of the firstreceived frame address byte must compare successfully along with theother address bits for address recognition to occur by address detectionunit 226.

Bit 7: First/Second Byte Selection (Default=0)

This bit only has effect when one-byte addressing is selected when setto ONE the address recognition block 226 examines only the second byteof the address, i.e., the first eight bits are don't care. When clearedto ZERO, only the first byte is examined.

LINK ADDRESS RECOGNITION REGISTERS within blocks 278, 280, 282 and 284(FIG. 17).

Link Address Recognition Register 0 (278) (Default=Hex 0000)

Link Address Recognition Register 1 (280) (Default=Hex 0000)

Link Address Recognition Register 2 (282) (Default=Hex 0000)

Link Address Recognition Register 3 (284) (Default=Hex 0000).

All bits in these registers are set and cleared by software except wheninitialized to default values as a result of DLC Reset or IDPC Resetpin. These registers can be written and read by the local microprocessor18.

Link address recognition is defined in connection with FIG. 17. Each ofthese four registers has a corresponding enable bit (bits 0-3) in theDLC Address Control Register. If the corresponding enable bit is set, agiven Link Address Recognition Register is conditioned by bits 5 and 6of DLC Address Control Register as described hereinabove.

SERIAL BUS Port (SBP) CONTROL REGISTER is an eight-bit register. Allbits in this register are set and cleared by software except wheninitialized to default values as a result of DLC Reset or IDPC ResetPin. This register can be written and read by the local microprocessor18. ##STR3##

Bit 0: Invert (Default=0)

When this bit is set to ONE, the transmitted serial bit stream isinverted by XOR 200 (FIG. 8) as the last step following all other DLCtransmitter processing but before SBP channel multiplexing (block 196)(see bits 1-2 below). The one exception to this rule is when the DLCtransmitter 102 is transmitting the Mark Idle data pattern; in thiscase, no inversion is performed because Mark Idle is inserted by OR 202past the inverter 200.

When this bit is set to ONE, the received serial bit stream is invertedby XOR 238 (FIG. 11) as the first step following de-multiplexing (block232) (see bits 1-2 below) and Mark Idle detection. If Mark Idle isdetected, inversion continues to take place but no data enters the DLCreceiver Serial to Parallel Shift Register 212.

If this bit is cleared to ZERO, no data inversion takes place in eitherthe transmit or receive direction.

Bits 1-5: Channel Select (Default=00000)

Select SBP time slot for multiplexing transmitted serial bit stream bytime slot multiplexer 196 and de-multiplexing received serial bit streamby time-slot demultiplexer 232.

    ______________________________________                                        Bit   54321     Selection                                                     ______________________________________                                        00000       Channel 0  (Bd)*                                                  00001       Channel 1  (Be)*                                                  00010       Channel 2  (Bf)*                                                   .            .                                                                .            .                                                                .            .                                                               11110       Channel 30                                                        11111       Non-multiplexed -- single channel                                             with receiver clocked by SCLK pin and                                         transmitter clocked by SFS/XMIT Clock                                         pin.                                                              ______________________________________                                         *terminology employed in the crossreferenced application entitled "Digita     Subscriber Controller                                                    

Bit 6: Local Loopback Enable (Default=0)

When set to ONE this bit causes the transmit data path (SBOUT) to beconnected internally to the receive data path (SBIN). The selectedtransmit clock (either the SCLK or the SFS/XMITCLK clock) is used forboth the transmit and receive clock. The Local Loopback mode, whenselected, operates regardless of the setting of the Transmit Enable andReceive Enable bits (bits 1 and 2 of the Command/Control Register).Setting of this bit to a ONE also prevents data from being placed on theSBOUT pin or incoming data (from SBIN) from being received. Clearingthis bit to a ZERO disables Local Loopback.

Bit 7: Remote Loopback Enable (Default=0)

This bit when set to ONE, connects the SBIN pin to the SBOUT pin.Incoming data is therefore presented immediately to SBOUT as transmitdata. In this mode the appropriate receive clock is the SCLK. Receivedata may be presented to the DLC receive logic or not depending upon thesetting of Receive Enable. Data from the transmit logic is preventedfrom being sent out on SBOUT while in this mode. Clearing of this bit toZERO disables Remote Loopback.

MINIMUM RECEIVE PACKET SIZE REGISTER is an eight-bit register (264) FIG.15. Default=Hex 5. Bits 0-3 of this register are set and cleared bysoftware except when initialized to the default value of 5 as a resultof DLC Reset or IDPC Reset pin. Bits 4-7 are not used. This register canbe written and read by the local microprocessor 18.

This register indicates the minimum length packet (exclusive of Openingand Closing Flags) that can be received by the DLC without generating a"Short Frame" error in the Receive Frame Status Register.

At the time that the Short Frame Interrupt is generated, the contents ofthe Receive Byte Count Register reflects the number of bytes in theShort Frame.

    ______________________________________                                                Value Count                                                           ______________________________________                                                1     0001                                                                    .     .                                                                       .     .                                                                       .     .                                                                       15    1111                                                                    16    0000                                                            ______________________________________                                         ##STR4##

MAXIMUM RECEIVE PACKET SIZE REGISTER is a 16-bit register. Default=Hex0000.

The 16 bits of this register are set and cleared by software except wheninitialized to the default value as a result of DLC Reset or IDPC Resetpin. This register can be written and read by the local microprocessor18.

This register indicates the maximum length packet (exclusive of Openingand Closing Flags) that can be received by the DLC without generating a"Long Frame" error in the Receive Frame Status Register. As each packetbyte is received, the contents of the Maximum Receive Packet SizeRegister are compared with the Receive Byte Counter 292 (FIG. 18). Ifthe maximum packet size is exceeded in the Receive Byte Counter, a "LongFrame" error is generated in the Receive Frame Status Register. At thispoint, the received byte that caused the Receive Byte Counter 292 toexceed the maximum length is tagged as the End-of-Frame (EOF) byte andthe DLC receiver enters receiver state 0 (Hunt for Flag).

    ______________________________________                                        Value        Count                                                            ______________________________________                                        1            0000000000000001                                                 .            .                                                                .            .                                                                .            .                                                                65.535K      1111111111111111                                                 65.536K      0000000000000000                                                 ______________________________________                                         ##STR5##

DLC INTERRUPT ENABLE REGISTERS

Interrupt Source Interrupt Enable Register: (Default=Hex 0000)

Receive Frame Interrupt Enable Register: (Default=Hex 0000)

Receive Link Interrupt Enable Register: (Default=Hex 0000)

FIFO Status Interrupt Enable Register: (Default=Hex 0000)

All bits in these registers are set and cleared by software except wheninitialized to default values as a result of DLC Reset or IDPC ResetPin. These registers can be written and read by the local microprocessor18.

The last three enable registers are bit-for-bit images of thecorresponding Receive Frame Status Register, Receive Link StatusRegister, and FIFO Status Register, respectively, described hereinbelow.The Interrupt Source Interrupt Enable Register is an image of thecorresponding Interrupt Source Register described hereinbelow except forbits 0-2, which have no interrupts associated with them.

The last three enable registers form the lower level of a two-levelinterrupt enable mechanism used for the corresponding three statusregisters. The three bits in the Interrupt Source Interrupt EnableRegister that correspond to these three status registers form the higherlevel of the two-level enable mechanism. For example, in order for theShort Frame error interrupt to be enabled, the Short Frame bit must beset to one in the Receive Frame Interrupt Enable Register (lower-levelenable) AND the Receive Frame Status bit must be set to one in theInterrupt Source Interrupt Enable Register (higher-level enable).

When an event occurs that causes a bit to be set in one of the threestatus registers (reference the descriptions of the three statusregisters) and both levels of status interrupt enable are set to ONE,the DLC interrupt is generated and the bit for that register is set toONE in the DLC Interrupt Source Register. If the status register bit isset to ONE and either interrupt enable level is not enabled, nointerrupt is generated and the Interrupt Source Register bit for thatstatus register is not set to ONE. * The Transmit Threshold Reachedinterrupt described in connection with FIG. 5 (bit 2 in the FIFO Statusand Enable Registers) described hereinbelow, differs in the followingway. The threshold reached bit reflects the actual real time conditionof the FIFO (above, equal to or below the threshold). The interrupt,however, is generated only when the level in the FIFO falls to thethreshold level. This prevents the generation of an interrupt when theFIFO is empty because the transmitter is not in use.

A software read of any of the three status registers clears the statusregister to ZERO and clears the interrupt condition caused by bits beingset to ONE in that status register. A DLC Reset or activation of theIDPC Reset Pin also has this effect.

In contrast to the interrupt enable mechanism used for the three statusregisters, the Valid Packet Received and Valid Packet Sent bits in theInterrupt Source Register can generate an interrupt through a singlelevel interrupt enable mechanism. When an event occurs that causeseither of these bits to be set in the Interrupt Source Register, if thecorresponding Interrupt Source Interrupt Enable Register bit has beenset to one, the DLC interrupt is generated. When either of these twoInterrupt Source Register bits is set to one and the correspondinginterrupt enable register bit is not set to one, no interrupt isgenerated. ##STR6##

TRANSMIT BYTE COUNT REGISTER (152) is a 16-bit register.

Default=0

Bits 0-15 of this register are set and cleared by software except wheninitialized to the default value as a result of DLC Reset, IDPC ResetPin or when an Abort is issued by Flag/Abort Insertion Unit 134 andconducted via signal line 164 (FIG. 5). This register can be written orread by the local processor 18.

Software writes this register with the count of bytes to be transmittedin each frame, NOT including Opening Flag, Closing Flag, and FCS (CRC)bytes. Software only writes this register when the count of bytes to betransmitted is different from the count currently in this register.

The contents of this register are transferred to the Transmit ByteCounter 154 (FIG. 5) whenever software writes this register (if thetransmitter is out-of-frame) or when an End-of-Frame (EOF) tagged byteis loaded from the transmit FIFO 100 into the Parallel to Serial ShiftRegister 110. If software is writing this register when the EOF-taggedbyte is loaded, the transfer to the Transmit Byte Counter is delayeduntil the software write is complete.

    ______________________________________                                        Value                                                                         ______________________________________                                        1            0000000000000001                                                 .            .                                                                .            .                                                                .            .                                                                65.535K      1111111111111111                                                 65.536K      0000000000000000                                                 ______________________________________                                         ##STR7##

FIFO THRESHOLD REGISTER is an eight-bit register. The 8 bits of thisregister are set and cleared by software except when initialized to thedefault value as a result of DLC Reset or IDPC Reset. This register canbe written or read by the processor 18. ##STR8##

Bits 0-3: Transmit FIFO Threshold (Default=Hex 8)

As each byte of a packet being transmitted is transferred to the DLCtransmitter Parallel to Serial Shift Register 110, the contents of theTransmit FIFO Threshold bit field are compared by threshold comparisonlogic 185 with the count of bytes still present in the transmit FIFO150. The results of this comparison and their effect on TransmitThreshold Reached are discussed hereinbelow in connection with FIFOStatus Register.

The Transmit Threshold Reached signal is also used to condition the DLCtransmit DMA Data Request signal.

    ______________________________________                                               VALUE  COUNT                                                           ______________________________________                                                0     0000                                                                   .      .                                                                      .      .                                                                      .      .                                                                      15     1111                                                            ______________________________________                                    

Bits 4-7: Receive FIFO Threshold (Default=Hex 8)

As each byte of a packet being received is transferred from the DLCreceiver Serial to Parallel Shift Register 212 to the Receive FIFO 106,the contents of the Receive FIFO Threshold bit field are compared withthe count of bytes still present in the Receive FIFO by block 296 (FIG.18). The results of this comparison and their effect on ReceiveThreshold Reached are discussed in detail in hereinbelow in connectionwith the FIFO Status Register.

The receive FIFO threshold block 296 counts by two instead of by one, asin the Transmit FIFO threshold. This is because the receive FIFO is 32bytes deep, while the threshold bit field in the threshold register isonly four bits long.

    ______________________________________                                               VALUE  COUNT                                                           ______________________________________                                                2     0001                                                                   .      .                                                                      .      .                                                                      .      .                                                                      30     1111                                                                   32     0000                                                            ______________________________________                                    

INTERRUPT SOURCE REGISTER (620) is an eight-bit register. The InterruptSource Register contains the status information most important to theuser during normal operation. The intent of this register is to narrowdown what caused a DLC interrupt in as few steps as possible. Thisregister is a read only register.

This register is initialized to the default value shown below for eachindividual bit and bit field as a result of DLC Reset or IDPC Reset Pin.

Note: Bit 3 and Bit 5 are a special case of the four stage statusreporting mechanism described in connection with FIG. 27. These two bitsdo not need to be implemented in Stages 1, 2, and 3; instead, they canbe generated in Stage 4 from other Stage 4 bits as follows:

Bit 5 (Receive Frame Status) can be updated by DLC hardware as thelogical OR of the six bits of the Stage 4 Receive Frame Status Registerdescribed hereinbelow. Bit 3 (Valid Packet Received) can be updated asthe logical NOT of bit 5 at the same time bit 5 is updated. ##STR9##

Bits 0-2: Receive Link Address Field (Default=110 with LSB=0)

The Receive Link Address bit field is determined by DLC, addressdetection unit 226 whenever a frame is received (with or withouterrors). This bit field is one of the unique Four Stage Status Registersand Bit Field entities described in connection with FIG. 27.

This bit field is Stage 4 hardware and is therefore loaded (exceptduring DLC Reset or IDPC Reset) only when both of the following eventshave occurred:

1) Stage 4 is cleared by software read of this Interrupt Source Registerand

2) The End-of-Frame (EOF) tagged byte is read from the receive FIFO 290(by DMA or software).

The following table indicates which value is loaded into the ReceiveLink Address bit field as Stage 3 status is being loaded into Stage 4:

    ______________________________________                                        Bit                                                                           2   1     0      Meaning                                                      ______________________________________                                        0   0     0      Contents of Link Address Register 0 recognized               0   0     1      Contents of Link Address Register 1 recognized               0   1     0      Contents of Link Address Register 2 recognized               0   1     1      Contents of Link Address Register 3 recognized               1   0     0      Broadcast Link Address (all 1's) recognized                  1   0     1      Not used                                                     1   1     0      Default Value -- No packet received                          1   1     1      Packet received with no address recognition                                   enabled (i.e., bits 0-4 of DLC Address Control                                Register all cleared to zero)                                ______________________________________                                    

The Receive Link Address bit field is reset to its default value whenDLC Reset is executed or the IDPC Reset pin is activated.

Bit 3: Valid Packet Received (Default=0)

The Valid Packet Received bit is set to ONE by the Receive FIFO 106 whenthe End-of-Frame (EOF) tagged byte is read from the Receive FIFO DataRegister 298 (i.e., all packet bytes transferred to memory) and none ofthe receive errors described in connection with FIG. 20 has beendetected for that packet. This bit is gated when Stage 3 status isactually transferred to Stage 4 of the Four Stage Receive StatusRegisters and Bit Field.

This bit is cleared to ZERO when this register is read by software, DLCReset executed, or IDPC Reset pin activated.

Bit 4: Valid Packet Sent (Default=0)

This bit is set to ONE when the last bit before the Closing Flag hasbeen transmitted by the DLC transmitter 102 (i.e., Transmit ByteCounter=0 and no Underrun and transmitter Out of Frame).

This bit is cleared to ZERO when this register is read by software, DLCReset executed or IDPC Reset pin activated.

Bit 5: Receive Frame Status (Default=0)

This bit is set to ONE when any bit in the Receive Frame Status Register64 described hereinbelow is set and BOTH the corresponding bit set inthe Receive Frame Interrupt Enable Register AND the Receive Frame Statusbit set in the Interrupt Source Interrupt Enable Register.

This bit is gated when Stage 3 status is actually transferred to Stage 4as shown in FIG. 27.

This bit is cleared to ZERO when the Receive Frame Status Register isread by software, DLC Reset executed, or IDPC pin is activated.

Bit 6: FIFO Status (Default=0)

This bit is set to one when any bit in the FIFO Status Registerdescribed hereinbelow is set with BOTH the corresponding bit set in theFIFO Status Interrupt Enable Register AND the FIFO Status bit set in theInterrupt Source Interrupt Enable Register.

This bit is cleared to ZERO when the FIFO Status Register is read bysoftware, DLC Reset executed, or IDPC Reset pin activated.

Bit 7: Received Link Status (Default=0)

This bit is set to one when any bit in the Receive Link Status Registerdescribed hereinbelow is set with BOTH the corresponding bit set in theReceive Link Interrupt Enable Register AND the Receive Link Status bitset in the Interrupt Source Interrupt Enable Register.

This bit is cleared to ZERO when the Receive Link Status Register isread by software, DLC Reset executed, or IDPC Reset pin activated.

RECEIVE BYTE COUNT REGISTER (294) is a 16-bit register.

Default=0

This 16 bit register indicates the number of bytes received in a packet(i.e., between but not including Opening and Closing Flags) whether thepacket was received in error or not. The receive byte counter 292 isincremented when a byte of data is placed into the receive FIFO 290. TheReceive Byte Counter register is one of the unique Four Stage StatusRegisters and Bit Field entities described in connection with FIG. 27.This register is a Stage 4 register and is therefore loaded with a validbyte count only when both of the following events have occurred:

1) Stage 4 is cleared by a software read of this Receive Byte CountRegister and

2) The End-of-Frame (EOF) tagged byte is read from the receive FIFO 290(by DMA or software).

This register is reset to its default value of 0 when DLC Reset isexecuted or the IDPC Reset pin is activated. This register is a readonly register from the local processor 18. ##STR10##

RECEIVE FRAME STATUS REGISTER is an eight-bit register. The bits of thisregister are set to ONE as described below. Bits 6 and 7 are not used.This register is a read only register for local processor 18.

The setting of any bit in the Receive Frame Status register will set bit5 of the Interrupt Source Register if the corresponding enable bit isset in the Receive Frame Interrupt Enable Register and the Receive FrameStatus bit is set in the Interrupt Source Interrupt Enable Register asdescribed hereinbelow in connection with the Interrupt Source Register620.

The bits of the Receive Frame Status register are all cleared to ZERO(default bit settings) when DLC Reset is executed, the IDPC Reset pin isactivated or the register is read and the hardware condition that setthe bit no longer exists.

The register indicates which error or exception condition has occurredduring the reception of a frame. This register is one of the unique"Four Stage Status Registers and Bit Field" entities described inconnection with FIG. 27. This register is a Stage 4 register and istherefore loaded (except during DLC Reset or IDPC Reset) only when bothof the following events have occurred:

1. Stage 4 is cleared by software read of this Receive Frame StatusRegister and

2. The End-of-Frame (EOF) tagged byte is read from the receive FIFO 290(by DMA or software).

The Receive Frame Interrupt Enable Register is a bit-for-bit image ofthe Receive Frame Status register. ##STR11##

The setting of a bit in the Receive Frame Status register is establishedat Stage 1 and propagates through to this Stage 4 register. The tablebelow indicates the precedence of the various errors and exceptionconditions flagged by this register, listed in descending order ofprecedence:

    ______________________________________                                        Bit          Name                                                             ______________________________________                                        0            Abort Received                                                   5            Overrun                                                          3            Short Frame                                                      4            Long Frame                                                       1            Non-lnteger Number of Bytes                                      2            CRC Error                                                        ______________________________________                                    

If, following a read to the Interrupt Source Register, the Receive FrameStatus Register is not read, and it is not normally read for a validpacket, before the LSB of the Receive Byte Count Register, reading theReceive Byte Counter Register will clear the Receive FrameSynchronization Register. This maintains synchronization of the registerstack, i.e., stages 1-4.

Bit 0: Abort Received (Default=0)

This bit is set to ONE in Stage 1 (and eventually propagated to Stage 4)as a result of the DLC receiver Abort Detector 214 detecting an Abortcharacter (7 ones while In-frame) while the DLC receiver is In-frame andat least 3 bytes have been received.

Bit 1: Non-Integer Number of Bytes Received (Default=0)

This bit is set to ONE as a result of the DLC receiver Flag Detector 214detecting a Closing Flag character with at least 3 bytes received when anon-integer number of bytes has been received in a non-Short Frame(i.e., at least one but less than eight bits were received after ZeroBit Deletion in the byte immediately preceding the Closing Flag).

Bit 2: CRC Error (Default=0)

This bit is set to ONE as a result of the DLC CRC Checker 222 detectingan error when CRC Check is enabled in the DLC Command/Control Register.

Bit 3: Short Frame Error (Default=0)

This bit is set to ONE as a result of the DLC receiver detecting a ShortFrame error by short frame byte counter 260.

Bit 4: Long Frame Error (Default=0)

This bit is set to ONE as a result of the DLC receiver detecting a LongFrame error by the Receive Byte Counter 292 in conjunction with theMaximum Receive Packet Size REgister described hereinabove.

Bit 5: Overrun Error (Default=0)

This bit is set to ONE as a result of the DLC receive FIFO 290 detectingan Overrun condition described in connection with FIG. 16, i.e., thereceive FIFO 290 contains 16 bytes when received data needs to move intothe FIFO from the Serial to Parallel Shift Register.

RECEIVE LINK STATUS REGISTER. Each of the bits in this register areindividually set to ONE or cleared to ZERO by the DLC to indicate thereal-time state of the various status conditions they represent. Bits3-7 are not used. Setting any bit in this register will set bit 7 of theInterrupt Source Register if the corresponding enable bit is set in theReceive Link Enable Register and the Receive Link Status bit is set inthe Interrupt Source Interrupt Enable Register. Upon completion of theDLC Reset or IDPC Reset, the bits in the Receive Link Status registerwill be set/cleared to their default condition as a result of theResetting of the portion of the DLC that they monitor.

The Receive Link Interrupt Enable Register described hereinabove is abit-for-bit image of this register. ##STR12##

Bit 0: Mark Idle (Default=0)

This bit is set to ONE when the DLC receiver Mark Idle Detector senses aMark Idle data pattern (15 ones). This bit is cleared to 0 when thefirst 0 bit is detected on the receive data link.

Bit 1: Flag Idle (Default=0)

This bit is set to ONE when the DLC receiver Flag/Abort Detector 214senses two or more Flag characters when not In-frame. It is cleared toZERO when the first non-Flag character is detected by block 214.

Bit 2: In-Frame (Default=0)

This bit is set to ONE when the Flag/Abort Detection Unit 214 of the DLCreceiver 108 detects an Opening Flag followed by a non-Flag, non-Abortcharacter. This bit is cleared upon receipt of a Closing Flag (Flagreceived while In-frame) or any exception condition that terminates theframe abnormally.

FIFO STATUS REGISTER: Each of the bits in the FIFO Status register areset to ONE or cleared to ZERO by the DLC to indicate the real-time stateof the various status conditions they represent. Bits 5-7 are not used.

Upon completion of DLC Reset or IDPC Reset pin, the bits in thisregister will be set cleared to their default condition as a result ofthe Resetting of the portion of the DLC that they monitor.

The FIFO Status Interrupt Enable Register described hereinabove is abit-for-bit image of the FIFO Status register. Setting any bit in theFIFO Status register will set bit 6 of the Interrupt Source Register ifthe corresponding enable bit is set in the FIFO Status InterruptRegister and the FIFO Status bit is set in the Interrupt SourceInterrupt Enable Register. ##STR13##

Bit 0: Receive Threshold Reached (Default=0)

This bit is set to ONE when the number of bytes in the DLC receive FIFO290 becomes greater than or equal to the count in the Receive FIFOThreshold bit field of the DLC FIFO Threshold Reached Logic 296. Thisbit is cleared to ZERO when the count of bytes in the receive FIFObecomes less than the Receive FIFO Threshold bit field.

Bit 1: Receive FIFO Data Available (Default=0)

This bit is set to ONE whenever there is a byte available to be readfrom the DLC Receive FIFO Data Register 298 as indicated by the DataAvailable signal generated thereby. This bit is cleared to ZERO when abyte is read from the Receive FIFO Data Register 298 and the FIFO buffer290 location just above the Data Register is empty. The bit is clearedwhen the last byte of a packet is read from the Receive FIFO 290. It isnot re-enabled until the user reads the LSB of the Receive Byte CountRegister. This provides an indication of when the last byte of a packethas been read.

Bit 2: Transmit Threshold Reached (Default=0)

This bit is set to ONE when the number of bytes in the DLC transmit FIFO100 as monitored by Transmit Byte Counter 154 becomes less than or equalto the count in the Transmit FIFO Threshold bit field of the DLC FIFOThreshold Register. This bit is cleared to ZERO when the count of bytesin the transmit FIFO becomes greater than the Transmit FIFO Thresholdbit field.

Bit 3: FIFO Buffer Available (Default=1)

This bit is set to ONE whenever the DLC Transmit FIFO Data Register isempty (i.e., available to be written). On a write, this bit will remainactive if the FIFO transmit buffer 150 location directly above the dataregister 160 is empty. The bit is cleared when the last byte of a packetis in the FIFO buffer 150 (EOP tag). This prevents multiple packets fromexisting in the FIFO buffer 150 at the same time.

Bit 4: Transmitter Underrun (Default=0)

this bit is set to ONE if the output location of the Transmit FIFObuffer 150 (opposite end of the FIFO from the FIFO Data Register 160) isempty when a transmitter Parallel to Serial Shift Register 110 load isattempted. The Transmit Byte Counter 154 is implicitly nonzero for thisload to be attempted. A discussion in connection with FIG. 5 is relatedto the Transmit Byte Count Register 152.

Bit 5: EOP In Receive FIFO (Default=0)

This bit, when set to ONE, indicates that the last byte of a packet hasbeen loaded into the receive FIFO 290. The bit remains set until thereare no EOP tags in the FIFO 290.

Bits 6-7: Not Used 4.4.1.15

DLC FIFO DATA REGISTERS

DLC Receive FIFO Data Register 298.

DLC Transmit FIFO Data Register 160.

Each of these registers is eight bits long.

The receive FIFO Data Register 298 is read by DMA or software to removeone byte from the Receive FIFO 290.

The Transmit FIFO Data Register 160 is written by DMA or software toload one byte to the Transmit FIFO 150. ##STR14##

Bits 2-0

The three Received Bit Residue bits form a read only field thatindicates that number of received residue bits in the packet. Thedefault at Reset is all zeros. This field is cleared on either a read ofthe register, or a read of the LSB of the Receiver Byte Counter

    ______________________________________                                        CODE        BITS RECEIVED                                                     ______________________________________                                        000         8                                                                 001         1                                                                 .           .                                                                 .           .                                                                 .           .                                                                 111         7                                                                 ______________________________________                                    

Bits 5-3

The transmit Residue Bit Count field allows the user to specify thenumber of bits to be transmitted in the last byte of the packet (data isloaded into the transmit FIFO in byte quantities). This is a read/writefield, that is cleared by software, and defaults to all zeros at Reset.

    ______________________________________                                        CODE       BITS TRANSMITTED                                                   ______________________________________                                        000        8                                                                  001        1                                                                  .          .                                                                  .          .                                                                  .          .                                                                  111        7                                                                  ______________________________________                                    

APPENDIX B UART 54 Status/Control Registers (408)

RECEIVE FIFO DATA REGISTER 404a: The receive FIFO Data Register 404a(FIG. 21) (read only) is the output side of the receive FIFO. Datareceived by the UART 54 is read from the Receive FIFO 404, by theprocessor 18. The register is eight bits wide. Bit 0 corresponds to theleast significant data bit, which is the first bit to be transmitted orreceived. The default value at Reset is all ZEROs.

TRANSMIT FIFO DATA REGISTER 424a: The Transmit FIFO Data Register 424a(FIG. 21) is a write only input to the transmit FIFO 424. Data placed inthis eight bit wide register is transmitted out of the transmit FIFO 424least significant bit first (bit 0). The default value at Reset is allZEROs.

BAUD RATE DIVISOR LEAST-SIGNIFICANT BYTE AND MOST-SIGNIFICANT BYTEREGISTERS: These two eight-bit registers contain the low order and highorder eight bits of the number by which the UART clock input (UARTCLK)is to be divided by Baud Rate Generator 414. Bit 0 of each register isthe least significant bit of each byte. The default value at Reset isall ZEROs. When the two registers are combined the divisor is asfollows:

    ______________________________________                                        0000000000000001 = DIVIDE BY 1                                                .                                                                             .                                                                             1111111111111111 = DIVIDE BY 65535                                            0000000000000000 = DIVIDE BY 65536                                            ______________________________________                                    

Divide by 1 passes the UARTCLK through unaffected. This allows thereceiver and transmitter to operate off of separate external clocks.

Any write to either the MSB or LSB Divisor Registers causes the baudrate generator 414 to be loaded with the 16-bit value stored in the BaudRate Divisor Registers.

INTERRUPT ENABLE REGISTER: The Interrupt Enable Register is an eight-bitread write register used to enable specific interrupt sources. Setting aspecific bit to ONE enables its corresponding interrupt. The defaultvalue at Reset is all ZEROs. Resetting a bit to ZERO disables theinterrupt and resets the interrupt pin if the corresponding condition ispresent.

    __________________________________________________________________________     ##STR15##                                                                    BIT             INTERRUPT SOURCE                                              __________________________________________________________________________    0               Receive FIFO 404 threshold reached                            1               Transmit FIFO 424 threshold reached                           2               Receiver line status: overrun, parity, break,                                 framing (412)                                                 3               Modem status: CTS, DSR                                        4               UART status: receive FIFO timeout                             5               UART status: Special character received (412)                 6               Transmit line status: transmit shift register                                 420 empty                                                     7               Not used                                                      __________________________________________________________________________     *Not present in the 8250                                                 

INTERRUPT IDENTIFICATION REGISTER. This is a four-bit read-only registerused to identify which UART status register contains an interruptingcondition. Unused bit positions (7-4) contain ZEROs when this registeris read. ##STR16##

BIT 0: The interrupts pending bit is cleared to ZERO if any interrupt ispending. The default value at Reset is ONE.

BITS 3-1: This field identifies the highest priority source of allinterrupts. The default value at Reset is all ZEROs.

    ______________________________________                                        BIT   PRI-                                                                    3 2 1 ORITY    SOURCE        RESET BY                                         ______________________________________                                        0 0 0 Fourth   CTS, or DSR   Reading the Modem                                                             Status Register                                  0 0 1 Third    Transmit FIFO 424                                                                           Reading this register                                           Threshold Reached                                                                           AND int. source = 001                            0 1 0 Second   Receive FIFO 404                                                                            Reading this register                                           Threshold Reached                                                                           AND int. source = 010                            0 1 1 First**  Overrun, Parity,                                                                            Reading Line Status                                             Special Character                                                                           Register                                                        Received,                                                                     Framing, or Break                                              1 0 0 Fifth*   Receive FIFO 404                                                                            Reading UART Status                                             Timeout       Register                                         1 0 1 Sixth*   Transmit shift                                                                              Reading this register                                           register 420 Empty                                                                          AND int. source = 101                            ______________________________________                                         *Not present in the 8250                                                      **Simultaneous receipt of a special character or a character with a parit     error, and a threshold reached condition must cause the interrupt request     to be generated for the special character or parity error prior to the        generation of the threshold reached interrupt.                           

BITS 7-4 Not used=0

LINE CONTROL REGISTER: The eight-bit Line Control Register is used toallow microprocessor 18 to program the serial interface parameters, aswell as to request that a break condition be transmitted. The defaultvalue at Reset is all ZEROs. ##STR17##

BIT: 1,0: Bits 0 and 1 define the character length.

    ______________________________________                                               BIT  1 0         LENGTH                                                ______________________________________                                                  0 0       5                                                                   0 1       6                                                                   1 0       7                                                                   1 1       8                                                         ______________________________________                                    

2: Bit 2 defines the number of stop bits. A 0 selects one stop bit, a 1selects either 1.5 stop bits for five bit characters, or 2 stop bits forsix, seven, or eight bit characters.

3: Bit 3 enables parity generation and checking, when set.

4: Bit 4 selects between even and odd parity; even, when set.

5: When bits 5 and 3 are set, parity is transmitted in the stateopposite that indicated by bit 4.

6: Bit 6 is used to request that a break condition be sent. The UART 54will send the break pattern whenever bit 6 is set (sent after currentcharacter has been transmitted). The shift register and transmit FIFOcontents are also discarded. The line will return to normal operationwhen the bit is cleared.

7: The Division Latch Access Bit must be set in order to access the BaudRate Divisor Registers, and cleared to access the Receive and TransmitFIFO Data Registers, and the Interrupt Enable Register.

MODEM CONTROL REGISTER: The five bit Modem Control Register allows theCPU to manipulate the link handshake signals. Additionally, the UART canbe placed in a Loop Back mode for testing. Unused bits (7-5) should beZEROs when the register is read. The default value at Reset is allZEROs.

    __________________________________________________________________________     ##STR18##                                                                    BIT            FUNCTION                                                       __________________________________________________________________________    0              Places DTR/ in its active state (low) when set.                1              Places RTS/ in its active state (low) when set.                2              Places Output1/ in its active (low) state when                                set. This is a general purpose control pin.                    3              Places Output2/ in its active (low) state when                                set. This is a general purpose output pin.                     4              Places the UART in a Local Loop Back condition.                5              Not used = 0                                                   6              Not used = 0                                                   7              Not used = 0                                                   __________________________________________________________________________     *These bits may be read and written in the IDPC10, but do not affect the      state of any pins. They must be labeled "RESERVED" in the IDPC10 data         sheet since they are not pinned out.                                     

LINE STATUS REGISTER: The eight bit Line Status Register contains flagbits, that when set, indicate the presence of a condition that whenANDed with the appropriate interrupt enable bits of the InterruptIdentification Register can generate a line status interrupt. Bits 1, 2,3, 4, and 7 are cleared by reading the Line Status register. Bit 5 iscleared when the condition goes away, but the interrupt is cleared byreading the Interrupt Identification Register (when the identificationregister is reporting this interrupt). Bits 0, and 6 are cleared whenthe causing conditions is no longer present. The default values at Resetare shown below.

    __________________________________________________________________________     ##STR19##                                                                    BIT       FUNCTION                                                            __________________________________________________________________________    0         Receive data available in Receive FIFO Data Register                          404a. Cleared when receive FIFO 404 empty. Default =                          0                                                                   1         Overrun error from Receive FIFO 404 (received data                            lost). Default = 0                                                  2         Receive Parity error detected by block 412. Default =                         0                                                                   3         Framing error (invalid stop bit) detected by block                            412. Character with framing error is not loaded into                          the receive FIFO 404. Default = 0                                   4         Break condition detected by block 412. Default = 0                  5         Transmit FIFO 424 threshold reached. Cleared when FIFO                        level rises above threshold. Default = 1                            6         Transmit shift register 420 empty (last character                             sent). Cleared when FIFO 424 and shift register 420                           are no longer empty. Default = 1                                    7         Special character received detect by block 412. Set                           when special character is loaded into the FIFO 404,                           cleared when Line Status Register is read. Default                  __________________________________________________________________________              = 0                                                                  *Not present in the 8250                                                 

MODEM STATUS REGISTER: The eight-bit Modem Status Register is used toindicate the condition of the link handshake input signals, as well asthe presence of a change in their status. Bits 3-0 default to 0 onReset, bits 7-4 reflect the input status.

    __________________________________________________________________________     ##STR20##                                                                    BIT         FUNCTION                                                          __________________________________________________________________________    0           Set if CTS/ has changed since this register was                               last read.                                                        1           Set if DSR/ has changed since this register was last                          read.                                                             2           Set by the trailing edge of Ring Indicate (RI/ on                             to off transition).                                               3           Set if Receive Line Signal Detect has changed since                           this register was last read.                                      4           Status of the CTS/ line (set if active - low).                    5           Status of the DSR/ line (set if active - low).                    6           Status of the RI/ line (set if active - low).                     7           Status of the RLSD/ line (set if active - low).                   __________________________________________________________________________     *Must be nonasserted in hardware in the IDPC10. These bits must be labele     "RESERVED" in the IDPC10 data sheet since they are not pinned out.       

UART CONTROL REGISTER: The eight bit UART Control Register is used tocontrol non-8250 like functions. Additionally, the UART software Resetbit is located here.

    __________________________________________________________________________     ##STR21##                                                                    BIT       FUNCTION                                                            __________________________________________________________________________    0         Receive clock MUX 410 select: set for internal baud                           rate generator 414, clear for external (RXCLK).                               Default at Reset = 0.                                               1         Transmit clock MUX 418 select: set for internal baud                          rate generator 414, clear for external (RXCLK).                               Default at Reset = 0.                                               2         Sync select: Set for synchronous, clear for                                   asynchronous. Default at Reset = 0.                                 3,4       Receiver FIFO 404 threshold reached is set when the                           number of bytes in the FIFO is equal to or greater                            than this level. Default at Reset =  11.                                        01 = 1                                                                        10 = 2                                                                        11 = 3                                                                        00 = 4                                                            5,6       Transmit FIFO 424 threshold reached is set when the                           number of bytes in the FIFO is less than or equal to                          this level. Default at Reset = 00.                                              00 = 0                                                                        01 = 1                                                                        10 = 2                                                                        11 = 3                                                            7         The UART 54 is Reset to its default condition when                            this bit is set by software. The Reset operation is                           identical to a hardware Reset via the RST pin. This                           bit is cleared by the Reset operation. Default                      __________________________________________________________________________              = 0.                                                            

UART STATUS REGISTER: The five-bit UART Status Register reports statusconditions that do not occur in an 8250 UART. Additionally, the"character with parity error available" bit is located in this register.The default value at Reset is all ZEROs, except bit 4 which is a ONE.Bit zero is cleared when the register is read. Bits 1-4 are cleared whenthe corresponding condition is no longer present.

    __________________________________________________________________________     ##STR22##                                                                    BIT          FUNCTION                                                         __________________________________________________________________________    0            Receive FIFO 404 timeout occurred, cleared when                               register read.                                                   1            Character with parity error detected by block 412 is                          available, set when a character with a parity error                           reaches the output of the receive FIFO 404a, cleared                          when character is read from the FIFO.                            2            Special character detected by block 412 is available,                         set when special character is at FIFO 404a output,                            cleared when character read from FIFO.                           3            Receive FIFO 404 Threshold Reached. Cleared when the                          number of bytes in the receive FIFO falls below                               the threshold level.                                             4            This bit is set whenever the transmit FIFO Data                               Register 424a is empty. This condition does not                               generate an interrupt. Default = 1                               5-7          Not used                                                         __________________________________________________________________________

What is claimed is:
 1. A bidirectional data communication port adaptable for use with a data link controller for controlling data communications with a station, the data communications port comprising transmitting means for transmitting outbound data to said station and receiving means for receiving inbound data from said station;said transmitting means including time slot multiplexer means for selectively establishing time-relations for said outbound data in response to a plurality of first clock signals; and said receiving means including a time segment demultiplexer means for selectively establishing time-correspondences for said inbound data in response to a plurality of second clock signals.
 2. A bidirectional data communication port as recited in claim 1 wherein at least a first of said plurality of first clock signals causes said time slot multiplexer means to establish a first time-relation by which said outbound data is transmitted conforming to time slots, said time slots being determined by said at least a first of said plurality of first clock signals; andat least a second of said plurality of first clock signals causes said time slot multiplexer means to establish a second time-relation by which said outbound data is transmitted at a continuous transmission rate for a transmission duration, said transmission rate and said transmission duration being determined by said at least a second of said plurality of first clock signals.
 3. A bidirectional data communication port as recited in claim 1 wherein at least a first of said plurality of second clock signals causes said time segment demultiplexer means to establish a first time-correspondence by which said inbound data is received during time segments, said time segments being determined by said at least a first of said plurality of second clock signals; andat least a second of said plurality of second clock signals causes said time segment demultiplexer means to establish a second time-correspondence by which said inbound data is received at a continuous reception rate for a reception duration, said reception rate and said reception duration being determined by said at least a second of said plurality of second clock signals.
 4. A bidirectional data communication port as recited in claim 2 wherein at least a first of said plurality of second clock signals causes said time segment demultiplexer means to establish a first time-correspondence by which said inbound data is received during time segments, said time segments being determined by said at least a first of said plurality of second clock signals; andat least a second of said plurality of second clock signals causes said time segment demultiplexer means to establish a second time-correspondence by which said inbound data is received at a continuous reception rate for a reception duration, said reception rate and said reception duration being determined by said at least a second of said plurality of second clock signals. 